Oh, balance....
[First of all, let me note that balance is a personal pet peeve of mine, and I'm generally ranting. I'm not targeting any specific person here with this rant.]
There's two kinds of balance.
The first kind is the balance between a party and the challenge the DM presents them with. If the game system provides the DM tools to be able to provide the right level of challenge to the party, a challenge that is difficult but achievable and neither a TPK or a cakewalk, then that's a good kind of balance. A game system that doesn't provide such tools, or provides tools that don't work very well, at the very least does make more work for the DM and could even be considered to be "broken". The word "broken" is overused quite a lot, though, in my opinion.
Then there's the second kind of balance, the intra-party kind, the balance of PC against PC. That kind of "balance" annoys the $#!! out of me, because frankly it has nothing to do with the game system. It's all about the kind of players you have. A player doesn't hog the spotlight because his character is more powerful, he hogs the spotlight because that's the kind of player he is. Building a powerful character is just one means of doing this, but it's the means the player uses to hog the spotlight, not the cause of it. With a lesser-powered character or in a different system with less mechanics, the same player would most likely use different means - e.g. excessively histrionic theatrics - to achieve the same end of hoging the attention. This kind of player has a need - perhaps compulsively, perhaps even pathologically - to be the center of attention, so the cause of this is psychological and has nothing to do with the mechanics of the game system. This kind of player, in the words of grade school report cards, "does not play well with others."
Differing levels of power within a group does not automatically result in spotlight hogging, if the players are mature enough to handle it. The campaign I play in has characters of widely varying power levels, because it's a long-running campaign that has progressed to the second generation of PC's. Last year, one of my first-generation characters, a Wiz20/Clr10, went out on a mission with a number of the second-generation PC's that averaged about 5th level or so. My 1st-genner was personally interested in the 2nd-genners; some were her children, and some were such close friends of the family that she helped raise them and were the next thing to stepchildren to her. It was a close-knit family group. I did not hog the spotlight with the character; she let the kids take the lead to get the mission done and concentrated mainly on healing them and keeping them alive. At the climax of the mission, the DM provided my 1st-genner with an evil leader that was quite a challenge and let the 2nd-genners take on the minions. As it turned out, my 1st-genner was the only one that got seriously hurt. But all the characters had their roles to play and no one hogged the spotlight. That's what happens, despite any difference in relative power levels, when you have mature players who work well as a team. Now, the next adventure we're looking forward to will have this same 1st-genner as well as some others. Two of the others are about 40th-character level, while three others are at about 16th. I fully expect it to be a good adventure despite this disparity in levels.
There's another kind of player personality disorder that invokes intra-party "balance", and this one is even more annoying to me - the one who does it out of some philosophical principle of egalitarianism he brings to the table. This kind of player has some misguided and overzealous adherence to egalitarianism as a principle in that every PC has to be as nearly equal in power to all the others, and this player obsesses about what other players' characters have in comaprison to his - e.g. "He's got a +3 sword and I've only got a +1!" or "his character can kick more butt than mine!". Again, such envy and jealousy on the part of the player is an issue with the player, not the game system. Perhaps it's just a meta-game means of hogging the spotlight, or perhaps they are bringing their real-life philosophies or politics to the table (but I won't explore the latter here), but in any case envy and jealousy is just as ugly at the gaming table as it is in any other social setting and when I see it I get really annoyed at the whiny player.
And as far as classes being supposedly unbalanced... well, who cares? I don't pick a class to play based on its relative power level. If I play a ranger, it's not because I think it is more powerful than another class or at least not less powerful, it's simply that I want to play a ranger. If I want to play a ranger, if the current character concept that holds my interest is that of a ranger, then I'm going to play a ranger regardless of whether the ranger class is more, equally, or less poweful than some other class.
If there's an issue of PC vs. PC balance at your gaming table, it doesn't mean the game system is broken or necessarily unbalanced, it just means you might need new players who don't whine so much and play well with others as a team.
I should know. heh. i'm still new at 3.5/3.0 rules and still get majorly confused at times. It is said by the game-makers themselves, the game can be played without the core books or dice or anything. (I personally don't know about that) 
