Nils from A Blog about Everything has an interesting design document for a MMORPG, in which he proposes (near the end of a long wall of text) to design a game in which magic was rare and powerful. Normally that would be a problem, because if there is a mage class more powerful than non-magic classes, lots of players would want to play it, and thus it wouldn't be rare any more. So he proposes to balance the class by having it have perma-death. Every other class would resurrect on dying, but mages would be permanently dead, and would have to reroll every time they die.
I don't know if that is such a good idea. If I look at World of Warcraft, I notice that my characters die rarely when soloing, and much more often in group situations. Perma-death would discourage players from joining groups, as they'd feel much safer alone. Nils has some proposals to overcome that, but I'm not sure if they are sufficient.
Would you want to play a character class which is much more powerful than the others, but had a huge disadvantage like perma-death?