Atmosphere. Using eyecandy, lightning techniques and possibly music to set a strong mood for the level and prevent it from getting boring. Combine that with a strategic, yet somewhat open and fast paced layout so games will require strategy but not go into deadlocks or long periods of nothingness. Maybe a few gimmicks to impress some people (although don't focus overly on them, and don't try to show them off).

Possibly make use of a variety of different events that might seem otherwise useless, and distribute all that along the level evenly to prevent certain areas from looking like they were just quickly tacked on.

Basically, a level that feels and looks great and uses interesting tricks, yet doesn't depend on that entirely and has a strong layout and ensure it is not forgotten.

That's pretty much a general view I guess. Personally, a level that uses events for eyecandy purposes (pinball bumpers, or chesire cats like in the Uninspired Psych Level) always make the levels seem more alive for me if they are done properly (maybe even try using roll rocks too, if they work? ;\). I like dark atmospheres, so that appeals greately to me (as does nice lighting use), although I'm sure that doesn't go for everyone. Always try to put things in open spaces, whether it be eyecandy or interesting pickup patterns, anything that prevents the area looking like the author got lazy, I tend to notice that I guess.

I don't know if that helped or not but ;D


edit: for things you should avoid I guess i would mention lightning. I hate constant flashes (ESPECIALLY if they're in the foreground. They just cause headaches) and they tend to make me want to turn low detail on, no matter how good the eyecandy is otherwise. If they're in the backround and there's lots of bg layer eyecandy to cover them up mostly, I guess it's alright. But definately not in the foreground, ugh.