Let's talk about party balance. Party balance in a table top RPG is vastly different than in fiction. In television, you have theoretical party balance, with the main hero overshadowing the rest of the party. No one really thinks the Ronin Warriors are a balanced party. In single player games, the hero can overshadow his friends. But, as you add players, particularly MMOs, the entire content system begins to revolve around the idea of a balanced party taking on level appropriate challenges (or, raw, overpowered cheese slicing through, uh, butter... look, this got away from me). D&D has been doing this for years, with crippling over specialization common in party dynamics You need to login to view this link .
Which is fine, for fiction. But in a game, if one person is T.G. Cid and the other is Lavian, the game will come to a grinding halt for Lavian in combat. Because, honestly, that's where party balance is most visible. And that, my friendly GMs, is where there is a second layer of party balance. What your team does out of combat requires party balance too, which Call of Cthulhu and some other games try to accomplish through a complex skill system. But, your barbarian probably feels kind of useless at a dinner party. I mean, what is Gau going to say to Emperor Gestahl, after all? The answer: Gau!
Balancing combat encounters is one thing; create varied threats so that the healer and the mezzer feel useful. Give rows of mooks for your Sorc to blast while the fighter locks down the ogre in melee. That's basic combat balance. Guild Wars 2 is doing combat balance by letting people bring a limited suite of abilities with discrete effects that combine well. As my guardian, I dropped a nifty aura on the ground to aid my team while smacking things, letting me participate as opposed to being a purely reactive support. Compare a World of Warcraft Priest's healing with the active role a Protection Monk has in Guild Wars 1. The more active a player feels, the more powerful they feel, even if their abilities are not, statistically, powerful.
Outside of combat is where things get harder. For example, in 3.5 and 4E, any non-combat encounter is pretty much dominated by the party face, every knowledge roll goes to the team scholar. If the face is a bard, then he probably monopolizes both of those roles in your group.