Eric at Elder Game is just winning all over the place in a post that covers a variety of balance topics. Two sample quotes:
I haven’t played much in years, but as far as I can tell, post Cataclysm, [WoW's] balancing plan has been to say “fuck solo balance, we’ll just make it insanely easy for all the classes, and then nobody will care enough to complain.” Which… is a valid approach for certain audiences. [Like me! -Sandra]
One of my rules of thumb is that an MMO shouldn’t balance gameplay via tedium. By that, I mean a designer shouldn’t say “Well you could become overpowered by doing X, but you’d have to do X for 500 hours, and who’s going to do that?” MMO players, that’s who. Because of the competitive environment, a lot of them will do whatever it takes, and they’ll curse your name for “making” them do it, all at the same time.
He goes on to explain how playing fetch will be used as a balance technique.
The second quote was an explicit part of balancing the original Magic: The Gathering. The developers balanced the game assuming a relatively small card environment. “This card is powerful, but it is rare, so that won’t be a problem unless people start buying thousands of cards. In which case, our game is a huge success, so no problem.”