Lyxen
Great Old One
I consider balance mostly a matter of spotlight time and general contribution. The game and party is balanced when these are generated just out of the normal course of play, without the GM having to force the issue with any frequency.
While I agree in general, I'm reacting to your post not specifically but as one of those referencing "spotlight". I used to think that way, but I've changed since I realised that this is just bias due to the projection of MY expectations on the game. But there are players (4e DMG referred to them as Watchers, but we've known the type ever since we started playing) who don't particularly seek the spotlight, indeed some people who would be inconfortable when pushed (too much) into the spotlight. And yet, these are perfectly legitimate players.
Same about contribution, actually, why should there be balance in the contribution either, our players are all different, with different expectations. At our tables, some players contribute a lot, others very little (not because they are being limited, just because it's how they play the game), does this make then lesser players ? Certainly not.
Hence my fall back to the only balancing factor that makes sense to me, way more than spotlight or contribution, it's just about having fun together, and the nice thing about fun, is that it can certainly be shared more easily than the spotlight.