I hear a lot of talk about how a wizard at high levels can pretty much demolish anything. While in theory , sure they can, but how many of you have actually DM'ed or played with someone like that? .
I've never had this problem as a DM, and I only had this problem once as a player... and the problematic character was my own!
I think this problem happens when the players are playing for themselves instead of having a team spirit, at least I realized that it was the problem for me: I was using all my spells to buff myself instead of spreading them to the whole team. Then of course I was very hard to kill, and my damage output in combat was the highest.
But is this the "correct" way to play D&D?
The answer is: if the result is not fun, then it means it is not the correct way to play!
The game became more balanced and more fun when I started to use more spells to benefit the other PCs depending on their needs.
Think about these two basic questions for instance:
1- Are you absolutely sure it is the best for everyone that a Cleric (who starts off as an inferior melee warrior that the fighter classes) buffs himself and become the best melee warrior of the party? Or does this mean you end up with the Cleric stepping on the Fighter's melee role, and having 2 good melee instead of having one decent (non-buffed Cleric) and one excellent (buffed Fighter)? Couldn't it be instead better to buff the best guy (Fighter) to excellent damage output and AC so that he can really be the 1st line of defense, and the Cleric is free to do more things?
2- Are you absolute sure it is the best for everyone that the Wizard becomes nearly-untouchable after self-buffing with protections from everything, when anyway she's still going to stay as far as possible from melee, when everybody else is dying on the front line due to low defenses?
In other words: does the Cleric or Wizard player "win the game" if she survives and everybody else dies (or suck) because she didn't share her buffs? Isn't she a loser instead?
Check out also the examples posted by other DMs above, and tell me if their players weren't playing mostly for themselves... Also note how many of those examples gravitate around the idea of turning their caster PCs into damage-dealers or melee tanks, which is not what their classes are meant to be. I don't think it's a "bug" that the game allows a spellcaster to become a better fighter than the Fighter. I think the "bug" is people playing D&D like a computer game, i.e. like a solo game, which D&D is not! It is a game of damn teamwork! Wouldn't it spoil the game if the Cleric would only heal himself? If the Rogue would only disarm traps for himself? So why should it be different for buffing?
Eventually a real problem instead lies in the versatility of spellcasters, particularly Wizards, because the higher the level, the wider the versatility gap with simpler classes who have less to do outside combat. At low/mid levels it's not an issue, but clearly if the Wizard can solve too many plot/adventuring issues with one of her spells, then there is certainly a problem...