I just want to share some knowledge and give some tips on mapping. This is for novice mappers as well as for people that are already familiar with Cube editing. For learning the basics of how to edit maps, refer to the readme.html.
Some opinions on certain topics might be influenced by my personal taste and views. I don´t claim this were the ways to do it, but it´s the way I do it and what I find important.

Since I have never made a singleplayer map (I like multiplayer better) I can only talk about making multiplayer maps here.
I will divide this into sections, which are

1. Layout/Performance
2. Gameplay
3. Design (detail, mapmodels, light and stuff)

1. Layout/Performance

Starting a new map, it is always good to have some basic concept or idea in mind.
Sometimes I will just start mapping and develop ideas as I go, but often I have an idea, and I will take a piece of paper and get it down first. (sometimes stanze aka lahme, my beloved girlfriend came up with sketches too ;)

So, while I build the basic layout of the map, I like to keep an eye on the performance, by carefully figuring out where solids are needed to occlude enough of the map to make it enyoyable on most systems.

Keep in mind that if you want the map to have a pretty large open area, you must sacrifice some of the detail, to keep the wqd count reasonable. By "reasonable" I mean about 2500-3500 average, tops may reach 5000-5500 but not much more.(the figures given in the readme.html have risen a little since 2001, but be sure to play aard3... it shows that a great map doesn´t need a high wqd-count) Even if you experience no slowdown on your system, please think about those people that don´t have such a pretty high-end machine like you have ;) A lot of people don´t only play cube because it´s cool and free, but also because it has relatively low system requrements.

Since working out the layout in fullbright, or with temporary 16 255 lights ("l"-button) will not represent the wqd counts of a fully detailed section of a map, I use to "spam" the whole map with small lights ("2 255") This way I can get a better picture of how much wqd a special section will have when fully detailed. (turn on "showmip" to see what I mean)

I add occluders where they seem neccesairy and this sometimes "dictates" the layout in a way, and it just helps to work out and realize the initial layout. This is yet another great thing about cube mapping. Also, a corridor that does a U- or S-turn of some sort, or has a pillar in the middle not only helps performance, but also makes fighting more intresting later !

While making the layout, I will try to alter the levels the map has (adding stairs or jumppads). In cube you can´t make a room over a room (with a ceiling dividing both),but if you add different levels to a map and do it good, noone will really notice that ;) Even just slight alterations can spice it up, look at metl3.

Also keep an eye on architecture itself. Of course, the places where cube happens in are by no means realistic, but some stuff might just look odd to the human eye. As an example, some 8 thick cube ceiling resting on 2 fragile 1 cube wide cube-pillars might look as if it´d tumble down every second.. also, a big building resting on an only 1 cube thick wall will look fragile aswell. And it would furthermore not leave enough room for intresring wall inlays too... in my maps, I try to keep major walls at least 4 cubes thick...
In this respect, it´s also a good idea to get the basic layout done first, rather than fully detailing out a room and then make the next one from scratch. At this state it´s still easy to change overall layout...

Let´s get to the basic idea behind the layout. There are basically 3 diffrent "classical" types of a Deathmatch-Map layout.

The "Arena" type layout : Maps like aard3, douze, tartech, nudist, kmap5 (and the like) are all centered around one main room/area, and have a rather circular layout. Where ever you go, in the end you will get back to the main area again. This type of layout is very popular, because you won´t get lost easily, and players will learn to navigate the map quite fast.

Next up is what can be called a "mazy" layout. Examples are biologie, mak1, powerplant, spillway, infertile, etc.. these maps are generally laid out for a larger number of players. Not being centered arount a main area these maps make sure a lot of individual fighting can commence, without all the players meeting in one place all the time. They rather consist of a series of larger rooms connected by lots of corridors.

The last type of layout is the "opposing bases" layout, that most ctf maps have. These maps are quite symetrical. Of course cube doesn´t have ctf (yet, hopefully), but on a smaller scale, this type of layout can be exellent for 1on1´s, or even for deatmatch. (see minion 1, ogrosupply)
The advantages are, that it´s easy to learn the map, and both players have just the same choices in where to go and where to predict/expect the enemy.

Of course, not all of cube´s maps can be categorized in these types, many are a mix of those. metl3 and metl2 are more a blend of "Arena" and "Mazy" style. After all, not everything you might have in mind will turn out very playable. Try to balance between idea/concept and actual playability.

You should know what you want gameplay to be like on your map, so it might help to keep these types in mind. This leads us to the most important element in a map:

2.Gameplay

Still a mystery to me and hard to figure out why a map is fun or not, I can only give some basic tips here.

Leave enough space !! This is very important.
Imagine a 50 meter long and 4 cubes wide, 6 cubes high corridor. You are running along it, trying to reach the next room with that valuable armour/health/ammo item in it. you run.. you run..
Half way though it, another player enters that corridor from behind, loaded up on ammo, good health and a big armour. The guy/girl starts shooting at you. What can you do? Evade his fire by strafing eradically? No, there´s no space to do that.. Evade his fire/rocket-splash by jumping like a mad kangaroo? Nope, you only bump against the ceiling.. turn around and answer fire? Yep, that´s all you can do, but you´re already dead...
See what I mean?

In that respect, try every corridor to have a good "headroom" so players can jump around. Many Cubers just love the high and bunny-like jumping it offers, and use it to be harder to hit, get less of those rocket´s splash damage and to confuse their opponent at close range.
Also, let narrow corridors be 8 cubes wide (or six, but no less) so players have a chance to strafe around... There are of course certain situations where this doesn´t have to apply, and it leads us to the next aspect of cube gameplay : Items!!

Item placement is very important for the fun a map can offer. I think there´s two types of items in cube. The ordinary ones (ammo, health,greenarmour) and the powerfull ones (healthboost, quad, yellowarmour)

Let´s take a look at the powerfull ones. These items are very important in order to survive long enough, or to get a lot of frags (quad). Therefore, it´s important to not have them too close to each other. If you make a room that holds the healthboost, quad and yellowarmour at once, gameplay will become a gamble. Whoever is gonna get to that room first can be the dominating player easily. Therefore, it´s better to scatter those items around the map, so everyone has a chance to get a piece of that sweet cake ;)
Also, don´t add too much of them. Only one of their kind helps to put the focus on certain areas. Some larger maps might hold 2 healthboosts or 2 greenarmours, but be careful. Do not oversupply the players, it could make it pointless to go to a certain place or not.

It´s also a good idea to place those important items in places that are a bit risky to reach. If everyone can grab ´em without effort, it´s just luck if you can get em or not.
This does mostly apply to the most powerful item, the quad. Look at how the quad is placed in metl1, metl2, douze, inkedskin, tartech... either you have to do a well planned jump (douze, metl1) or you have to go into a dead end (inkedskin/tartech,metl3) or you have to do special jumps like on metl2... this makes sure that getting the quad is some kind of risky, and that players seeing you getting it still have a chance to fight you (If they´re good).
It just helps to balance stuff, and make the map more challenging.

About the important items: I consider quad the most important, after that yellowarmour, then healthboost. Quad or not is also and important question. Usually a map with quad is more fun, one without is more appealing to the ambitious, competitive player. A quad in a map intended for 1on1 is basically not such a good idea. Good players can have a feeling of when to be there when the next quad spawns (some even count to 60), so the gameplay could rather end up in a "be there for the quad race" than some intresting and colourful fight.

MG ammo is a special case. It´s ammo, but by far the most important one. The machinegun in Cube is so powerfull that this is the most important weapon at all.
Ever seen how that ammo is fought over in several maps? Keep that in mind.

Ammo: Ammunition shouldn´t be distributed to generous. As the readme states, try to add it in a way that forces players to move around, and always keep an eye on it. Spamming one room with 4 bullet-entities, 2 packs of rockets, 2 health packs and even more ammo/poweups will invite players to just stay there and frag everyone comming by.
It´s also a good idea to put the certain types of ammo where they are most usefull. Put rockets somewhere above (because rockets work best when your opponent is somewhere below). Put shotgun shells where it´s narrow, put the rifle ammo where there´s a lot of space, for example at the start of a large corridor, or open area.
Put mg ammo wherever there´s large open or mid-distance situations nearby..
Just try to put the ammo where it makes sense for gameplay. This way you can also give hints to players (like "look this riflerounds, consider using it, because the next corner will lead you into some large area where you might find it usefull...")

Health should be scattered widely across a map. Try to put it so that players that need health will always know where to go to find the next health spot... also putting some just "along the way" and some in spots that have to be reached with some effort will spice up gameplay. (see metl3 for example)

However you put items in, I think the most important factor is to place them evenly and in a fair manner. If you compare nudist and inkedskin for example, these maps differ a lot when it comes to item placement. Nudist is really full of it, while inkedskin has a very spartanic item placement.
Both maps are fun though. Inkedskin is tough because you really have to keep an eye on your ammo and health. In nudist, you´ll run across it all the time, but still nudist is a very intense ffa experience.. both are perfectly balanced... ;)

Spawn Points:
Rule of thumb : The more, the better. Try to add as many as you can. Also, avoid spawnpoints that are in the middle of a room, or a corridor. Spawning and being instantly killed just because being in the line of fire of some other players is frustrating.
Note to novice mappers. It´s a common mistake to place the spawns with a wrong orientation.
If you want to add a spawn in a corner, select the cube you want the spawn to be and fly over it, then add the entity while looking in the desired direction. This holds true for teledest entitiys as well.

I have noticed that the spawnpoints you add first seem to be chosen more frequently for "random" spawning. So it´s a good idea to remove all temporary spawns you might have added while building the map, and then add the first ones to be far away from each other, like in the four courners of the map. This way you can make sure that players won´t spawn too much in one area, to a certain extent.

3. Design (detail, mapmodels, light)

Design is the sugar and spice of maps. The most important factor is to keep it on an even level.
Having one room just consisting of ceiling floor and walls, and the other full of heightfields, lamps, floor inlays and mapmodels, will make that room feel out of place.

When starting to work out the design and detail, it´s always good to have a theme in mind. Chose what´s going to be your prominent floor, wall and ceiling textures, and use the replace feature at first to get your map into a general theme. Also , orienting on the diffrent pack´s themes is a good idea. Some maps happen to combine a lot of styles (like cellar, exellent mix of themes) but it´s easier to go with those that fit together naturally (themes are generally "tech/base" (metl3, tartech, powerplant,mak2,ogrosupply, darth etc) "egypt" (nudist, gzdm1, metl1 etc) "midevial" (metl1, b2k, most maps that feature the new dg textures, and the "bricky"-kind (aard3, gib, inkedskin, douze...)

There are some diffrences between the textures of a certain pack. Some are to be used on a large scale, others arent, they are "trim" textures, and shouldn´t be used on large walls or floors, because they can become too repetitive.

It´s usually better go go around the map and add detail step by step. First you might concentrate on the basic Architecture, such as pillars, arches, the way your stairs and jumppads look. Try to find some good trim textures, and use em throughout the map. Add other "simple" architectural elements, like windows, additional arches or ceiling work, etc... go step by step, this will help to get a certain amount of consistency in design. While doing so, keep an eye on how wqd rises, to find out how detailed you can go.
After that, you can start to add diversity. Try to make certain areas recognizeable, either by adding unique elements, or by
slightly changing the textures used. Keep some things same throughout the map (preferred trims or lights/lamps) so a new room
will still feel like part of the same building.. This will help the player to learn the map faster, and to know where he is.
Bad examples are mak1 and q3dm2 both use far to many diffrent textures in my opinion. I don´t like em at all anymore...

Light: Important : do not use the "l" button everywhere. In cube, usually smaller light sources lead to better results, because they will add more contrast. Of course it´s faster to add some big light sources, but you can always bind some keys to "/newent light x x" ;)

Big light sources tend to blurr the textures and make it all appear a little bland. A more contrasting lightning will always be more visually appealing.
This is pretty much a matter of taste though. The most important rule should be (imho) that there has to be a visible diffrence between fullbright and lit.
Take a look at how maps you like are being lit, and learn from that.

Though cube´s lightning doesn´t produce shadows like they would naturally appear, you can still add a kind of realism to places. Outdoor areas can use bigger lightsources, while corridors and indoor rooms benefit from more contrast lightning. There are a lot of special "lamp" textures you can use on the ceilings and walls or even floors, in addition to small "highlights" they can add both detail and interesting light to indoor rooms...

Coloured lights: When adding coloured lights, be careful. Some textures can become bland if exposed to certain colours. If you add a yellow light to an egypt style map, the yellow will wash out the subtle shades of yellow/brown in the texture.
Also make sure you keep a consistent colour theme. Don´t add a red, a blue and a yellow light to the same area...
Good examples of coloured light are infertile (note how the red fog and red lights add to create a unique atmosphere), and minion1, where the slight blue in the lights help to crate a literally cool atmosphere and make the blue textures shine a little more, without washing them out) Metl3 is outstanding in using subtle colour to add fine variation to the textures.

Mapmodels:
You should add them after all is done, not sooner. After the 2004 release with all of dcp´s great mapmodels, there were a lot of maps done that used the mapmodels to add detail in a considerably bland environment. There are so many possibilities to add detail to a map without them. Be sure to have maxed out the architectural possibilities of cube(and your map) before adding them, only as "super extra spice". There´s nothing more boring than a map that looks empty after a "/clearents mapmodel"...

Be careful with the surrounding lights. Mapmodels will light up using only the amount of light of the cube they rest on. Try to adjust, especially when using architectural stuff. It might be hard to adjust a big pile of barrels to each other without
getting undesired effects on the sourrounding architecture, but you shouldn´t use large amounts of mapmodels anyways. Repetition will make look things boring.

Look at all the older maps, especially those from goetz and metl... it doesn´t take lots of mapmodels to make things interesting ;) And when you put them in, be sure to avoid repetition. They will lose their magic...

This is about all advice I can think of giving right now. Hope it helps somebody. Generally, take a look at your favourite maps and try to figure out what makes ´em so good, whether it´s design or gameplay.

You might also want to check out this intresting article:

http://www.cliffyb.com/art-sci-ld.html

and recently these two previews of chapters have popped up:
http://book.hourences.com/booklighting.htm
and
http://book.hourences.com/bookgameplay.htm