An imbalance in classes? The other classes were Druid, Beguiler, and Cleric. I don't think you'll find many stronger classes.
Imbalance in the encounter? It was a pre-made module, which meant that the default encounter should have been workable.
The problem was that the players used horrible spell selection and tactics. Now, you could argue that the DM should have softballed the encounter because of that, but he had no idea that the players would use poor strategy beforehand (obviously; he's not psychic) and he didn't pour over everyone's spell selection either.
Ok, that changes things.... You didn't share this info before. In fact, what you previously said:
I had a party that simply could not damage the hydra under the bridge towards the beginning of the game, so my sorcerer simply level drained it to death. This is an example of a optimized character ensuring party survival.
sounded like they were incapable of dealing with the encounter, and that only you (the optimized character) could deal effectively with the Hydra.
I'm sure you can understand the difference between this case and what i'm talking about in my previous posts.
Also, he was a busy college student and would not have had much time to adjust the module anyways.
A classic problem with DMs....time...
...especially when players are experienced and well prepared...
Now, I will agree that building characters to deliberately outshine other people is bad form, but Rumbletiger gave no indication of that. In fact, he seems to be helping rather than dominating, and I feel that this is an acceptable method of playing.
Well, as far as i can understand from what he wrote, he uses his character's 100% whenever things go pretty badly for the party...
This means that he can deal with regular encounters without a sweat.
He's the party's safety belt, and perhaps, the DM's safety belt as well.
Personally:
1-I wouldn't like being the savior. No challenge, no fun
2-I wouldn't like being any other player in the party. I'd feel...well better not say it...
3-I wouldn't allow this as a DM. Every body should get a piece of the pie. I would try to have a balanced party where good ideas and good choices/tactics is what gets the party out of serious trouble. Not some arithmetically perfect design.
I have encountered a couple of cases like this in the past.
In both cases there was a really powerful character in the party, who chose to stay low in general, but really outshined everybody else in the most difficult of encounters. Not because of his good tactics and ideas, but because of the his PCs overpowered abilities.
In one of the two cases. We discussed it, the DM acknowledged it, he even said how he felt somewhat comfortable because of this one PC. We all agreed and the powerful PC got an off-game downgrade.
In the second case, not even the DM liked it. We balanced things once again.
Players with the powerful PCs, did not like the idea of the downgrade, arguing how they do not use their 100% all the time, and one even argued how this is a part of his "roleplaying".
Perhaps they felt safe with such powers at hand...i can think of other reasons here... but there is no point of discussing them right now.
No matter how much those two PCs did not like it, no matter how they did not enjoy the game until their tilt was over...
the game got better, and the rest of the players enjoyed it more.
In each case it was 1 player's fun vs 3players' fun.