It's probably best if a level is fairly easy to learn, though it can still have a few secrets for players to find. This helps a lot with first impressions, which can mean a lot. All levels should have a good balance between open and relatively narrow spaces--an important element of atmosphere. I often go for some natural look when it comes to layout and eye candy, such as dirt ground only at the bottom of the level with structures on top. It's limiting, but I think it adds to the atmosphere. I also try to make different sections of a level look "unique" to aid in learning the layout, but I admit I don't always do a great job of this.

I still don't expect much from battle levels. I don't think a good gimmick is needed for them to be enjoyable, as opposed to CTF where there are many more quality levels. If it has decent eye candy and a good, balanced layout, I probably like the level.