I don't think @Arkandel was asking what an ideal power level is. He is looking for how much of a difference a game should allow between characters. The power level itself is irrelevant as it is all relative. The question isn't "Should all the characters be literal gods or should all of them be dung beetles," it's "Should some characters be literal gods while other characters are dung beetles?" At least that's my read on it.
Personally, if a game is going the free for all, no limit to xp earned and no limit to the number of maxxed out skills, then each increase in a skill should be as tiny as possible or you get the dinosaur problem. A person with a maxxed out skill in something should only defeat a person just starting off say 3 times out of 4. If you're keeping a very tight control on xp earned and limits on the number of maxxed out skills then a person with a maxxed out skill should defeat a person just starting off 9 out of 10 times.