A level with great eye candy is nice, but not everything a level should have. I would like to see an open level with a layout that makes it possible to not just run around in, but with some obstacles. In most Battle/CTF levels the lower part of the level is just a straight line.

Also, try to be creative with pickup placement. It's a bit hard to explain. But try to make shapes of ammo that look creative and don't look so old like this:

***
***

or

**
**

Try something like this:

** * *
* * *

In Battle levels, some food is also nice. It doesn't fit every tileset, but it's really fun in a Battle level.

Things you shouldn't put in:

-confusing eye candy

-very dark lighting(you can hardly see anything and that's no fun)

-too little ammo. It's fun if everyone has ammo, and not just a bit. You can put a few groups with much ammo, or you can put many groups with little ammo. So you shouldn't put too little ammo in.

-too many Power-Ups. I know they're fun, but putting them too close to each other in a small level is not fun. Myself I think 3 or 4 Power-Ups in a level is enough, really. I've seen small levels with 7/8 Power-Ups and that's just too much.

That's pretty much all I can think of so far.