RULE ZERO: At any time, I can deny your powers from either working or being accepted in the first place. I can also decide that 'that power doesn't work that way'.

Rule the first: Your power cannot be at wide ranging as a single element or concept.
No Fire elementalists or stuff like that. Tossing around fireballs is cool, but expanding that power to, say, turn into the human torch, would be another power entirely.

Rule the second: Powers that re redundant with established characters are fine.
Just as some people in real life are talented at the same thing or happen to look alike, genetic lottery can pull up the same power twice. There's no such thing as a power being 'taken'.

Rule the third: Powers do not give you free dominince over others.
You can freely make a 'mind controller', but he can only affect the 'weak-willed' (read: Non-powered npcs) or people who willingly submit. You can 'attack' other players, but it's like any other attack and is resistable.