Well, a few good features would be:
1) Automatic UV calculation
2) Grid snapping, making it easier and more accurate to create objects
3) light mapping, which can add dramatic elements to games
4) exporting to the DirectX file format, because that stores texture information, thusly making it easier to program games because you don't have to programmatacally texture it.
That's really all of the features that I would like in a level editor, and they're the most common in pro level design apps.
Signatures....peh. I never come up with anything good to say in them. Ah well.