Run with me on this. I just finished reading a great article on the Elder Game blog about how balancing in an MMO is ultimately a fruitless task. Then I thought back to another post I had read a while ago about the idea of Self-balancing systems. I have to come down on the idea that you simply can't balance everything. A truly "self-balancing" system would be probably more complex than the game itself.
But I did have one thought on how to try to... offset some of the players' expectations. I while ago I was thinking about how some many players complain about the random nature of a lot of MMO abilities. they all do random damage, and many have a random chance to "crit". When you get blown up by some mage who hit the jackpot in the lottery on his numbers and insta-gibbed it's pretty frustrating. Same for that mage who won't have that happen again for a week or a month or never. So why not take more of that randomness out of the random number generator, and put it into the environment?
What I mean is, make the effectiveness of abilities dependent on where and when you're fighting. If it's night time, an archer isn't going to hit quite as often, and maybe will get a critical hit even less often. During the winter, a character using frost-based magic will be a little stronger, while during the summer a fire-based character will be stronger. If there's a storm where you're fighting, maybe no one can get a critical hit because the wind is just blowing everyone around.
Not only natural things, but perhaps add some more supernatural effects as well. Deep underneath the ground are magical "Ley Lines" that follow paths that no one knows (but perhaps could figure out), and near them perhaps magic is stronger, or perhaps just more random.
Over time, the player community will figure out all these factors, and players will begin to consider their surroundings: "Oh why did I stupidly try to shoot down that warrior in this nighttime windstorm instead of just running?"; "There should be a Ley Line over there, I'll wait to ambush the caravan there."
The idea is that with a world that creates enough dynamic effects on how powerful each class is, players will notice a little less how "overpowered" certain classes or abilities might seem. Obviously this doesn't remove the need for balancing, but it might just lighten the load a tiny bit.