So: here's the long-awaited follow-up to the "I Hate Game Balance" rant.
When I'm feeling less ranty, I usually talk about the kind of balance I'm OK with: what I call "rough balance" or sometimes "internal balance", by which I mean that every class and every power has its downsides. It's not a universal "Win Game" button, even for a very narrow target. For example, there are always powerful undead that a particular cleric will have no chance of turning, and even autodestruction of undead can only affect 2d6 at a time. Or the general downsides to playing a magic-user: you start out with only a few spells, and even when you get to higher levels, you have to choose in advance what you think you will need, so sometimes you wind up not having the right tool for the job... and even when you do, your spells can be interrupted by injury, or a rival spellcaster could hit you with Feeblemind, etc. You don't always get what you want.
More specifically, each individual spell has its limits. Sleep can only affect creatures that sleep normally, and might not affect every target you want to affect (and, in some versions, might also affect allies or the caster.) Invisibility is great if you aren't planning on interacting with anything; it's like an automatic hide ability, as long as a creature primarily uses sight. But if you plan to attack something, really all the spell does is give you one free surprise attack.
But the thing about all these kinds of "balance", I realized, is that they aren't really balancing. What they do is enable things -- negative things -- to happen. And although the point of my rant was to explain that I hate discussions of game balance because they're full of a lot of nonsense and loose terminology, I've come to realize that I really do hate balanced games, not just because they tend to make all characters feel the same (as ProfessorOats mentions,) but because balancing characters against each other, or against the GM's opposition, is primarily about removing everything uncertain and interesting from the game.
Imagine joining an on-going game and being told you have to play one specific character, an invalid kobold accountant. All the other players are playing Clark Kent's relatives from the planet Krypton. Your typical game-balance fanatic would cite this as an example of why game balance is necessary, but the problem isn't game balance, it's the fact that someone has complete control over what you can choose.
And what the game-balance fanatics don't get is that removing uncertainty and making your character unstoppably perfect also removes choice.