You know, when I listen to players talking about how they enjoyed a game, never once have I heard them proclaim 'and the game was so balanced!' with a spark of joy in their eyes.
While I believe that balance is of critical importance, one thing to keep in mind is that balance isn't just about the system; the GM plays a very big role in balance themselves.
Sometimes you have to look at created character and just say, "The balance on this is broken." If the character is overpowered, you may have to talk to the player about changing some of their choices so that they're a little less perfect. If the character is underpowered, you can talk to the player about how to improve their character, or let them play that way if they're having fun and it's not detrimental to the group.
I realize that there are a lot of people who will scream bloody murder at the idea of forcing a player to take suboptimal choices for the sake of the game, but I will point out in a preemptive response that if one person is obviously more powerful, there are most likely others at the table who are losing some or all of their enjoyment of the game because of this.
One of the places where GM as balance really kicks in is when there are a lot of options. Some flavors of D&D gained so many supplemental books that it became easy to find broken and cheesy combinations. In a recent Mutants & Masterminds game I ran (which is point buy), one of the players was just significantly better at building a character than any of the others and chose purely optimal and mutually-supporting powers. He agreed not to do certain things with his character, such as adding teleportation, because it simply made his character unstoppable and it would have ruined the fun for everybody else. (Or would have simply eliminated the combat from the game.)
You know, when I listen to players talking about how they enjoyed a game, never once have I heard them proclaim 'and the game was so balanced!' with a spark of joy in their eyes.
They talk about their acts of daring do, about how they managed to get out of a sticky situation, even about how somebody had bribed a guard.
Balance is not a bugaboo, but it is not the be all and end all.
True. But I have listened to players talking about how a game disappointed them because of balance issues. I myself didn't feel like I had a very good time playing my TMNT-type character alongside another player's dragon character in Rifts.
And to borrow another's analogy: When I listen to people talking about how they enjoyed the cake my wife baked, never once have I heard them proclaim 'and the eggs you used were so fresh!' with a spark of joy in their eyes. But leave those eggs out...
Good point but I've also heard players complain about over balance or balance eating into believability.
I've read discussion of such on various messageboards, but I've never experienced that at the end of an actual session.
But I'm sure some players have. Like anything, too much or too little can cause problems.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.