To elaborate on the question in the title; what's that one level/section/area of a game you really like that stops you wanting to restart/replay the game at all?
I've got three personal picks to start the thread off myself:
Taris - Star Wars Knights of the Old Republic. - While not the most egregious example I just can't stand going through the place for about the fifth time. I've lost my data for my playthroughs beyond Taris on four separate occasions and I just can't face going through that place again. Yes, I'm aware of a mod to remove it, but that's not the point of this thread.
The Fade (Mage Tower) - Dragon Age Origins. - If you've ever played the game it seems like there's a chance you got filtered here too. I've quit on that section twice and even thinking about it kills my motivation to restart the game for a full playthrough. Mod thing applies here too.
The Arena - .hack// G.U. Trilogy. - Literal filler in every single entry of the trilogy. The fights aren't interesting or fun or even challenging, but they waste your time and I just can't be bothered with them again. Awful inclusion.