I just read the article at Gamasutra, MMO Class Design: Up With Hybrids! An Economic Argument in which the author analogizes the ecology of classes and player-competition within that to several economic factors. It was a good read. And it struck me that it would be really, really easy to provide one kind of balance to the class ecology by simply applying a reward multiplier (maybe just on XP if that's how your game rolls) based on nothing but the popularity of the class. If everyone detects that the fire/fire tank in CoH is uber and picks that, they might choose something else when they realize that the invul/mace tank earns xp at a rate 1.5 times as fast as the fire/fire. Right? And you could have that dynamically tuned as a weighted average based on the previous week on in-game play time or something. Then, barring stupendous gaffes in design which you've hopefully caught in testing, you can just completely avoid pissing people off when you nerf their class.
Is any game doing this? Why not?