The last 2 posts more or less covered gameplay, what to keep in mind while creating an environment of your own. A last simple hint for all you mappers out there is to keep the layout of a map SIMPLE. Having multiple routes for a Capture the Flag game sounds fun and all, but having to choose out of 500 different paths to take is overkill. Simplicity is key.

Simplicity is key for gameplay, but not neccessarily for the visual design around your layout. Let's break it down for the record. After you complete testing your layout, and you're satisfied with it, you start designing the visual side of things, meaning, you will want to try and make the map look good. The first term you'll encounter is theme. Theme is a group name for general architectural styles. Industrial, ancient and futuristic can indicate/define how a map looks. With industrial I mean concrete, wood, lots of metal, machinery, pipes, support, steam, and so on. Ancient has a temple like setting, with lots of stone, wood and murals. Futuristic has chrome, metal, and overall, a curvy feel. Obviously, my interpretations of these various themes aren't definitions of these themes, everyone has his/her own imagination, and these are by far not the only themes. Egyptian style architecture for instance, has grown to become a theme, while it would fit under Ancient.
So first, you choose a theme for your map. This all depends on what you're thinking at that moment, and what kind of things you're interested in. Own choice, no debate about that.

One of the most important parts for construction is part of the entire construction process and thus needs to be treated early on. I'm talking about optimization. Even though it's already explained in the gameplay article, it needs to be said here as well. Unreal engine works with various methods to keep the rendering process as smooth as possible. In other words, to have as many frames per second (or FPS in short) as possible. Some of these techniques, like use of semi-solids, can better be done during your map's construction than by applying them when everything's done already (this is also why I'm putting optimization info fairly early in this article).

First technique to use is zoning. What it does is basically dividing your level into various zones. The effect zoning has is that when you do not see ANY part of a certain zone, it won't get rendered. That's how big maps can run smooth, only the stuff you see, gets rendered. Zoning is done by using zoneportals. These are really just Sheet brushes, that 'close off' an area from the rest of the map. These are usually placed in doorways, for instance.
The second technique is antiportals. What antiportals do is occlude (not render) anything that's fully tucked away behind them. For instance, you place a big antiportal between rooms, that are connected by a single doorway. (note that Antiportals should NEVER be visible to a player ingame, so they should be placed in additive space). The rooms are zoned, but you're looking at the doorway, which makes the other room visible, thus it gets rendered. A lot of staticmeshes have been placed in the other room, and even though you cannot see most of them, they get rendered because they're in that specific zone. The antiportal between the 2 rooms occludes these staticmeshes (or any other type of actor, even brushes). The downside to this technique is that antiportals themselves cost rendering power. They should only be placed if the amount of occluded actors cost more rendering power than the antiportal itself.
Next is zonefog. Although it's used more for atmosphere than for optimization, zonfog can help out a bit. It's usually only used for optimization in ONS levels, as these are big enough. What zonefog does is create a coloured 'fog' that gets thicker over distance, which is fully configurable. At the point and beyond where the fog reaches 100% thickness, thus resulting in non-vision, stuff gets occluded.
CullDistancing! Another technique to use, but not usable that often, CullDistancing puts a limit of distance between the actor (mostly staticmeshes) and the player. When this distance limit is passed, the actor will no longer render. This should only be used occasionally as you may imagine, seeing stuff disappear while you're still able to see them isn't a good thing.
Collision is one of the biggest optimization tricks in the book as well. You have to make sure that all the staticmeshes used in your map have the simplest collision possible. Collision is a huge rendering power consumer, so the simpler the collision, the less rendering power needed. This especially goes for staticmeshes with high amounts of polys.