Final Fantasy XI by their reaction when someone mentions dunes. In Vana'diel, there is only one region known as the Dunes, and it is hated. Plagued with goblins whose sole purpose is to blow up low-level characters, ghosts that flay you alive with AoE spells fueled by nuclear energy and hate itself, and roaming bands of clueless players, the zone is not well-loved. All of the above might not be so bad, except that it's the most likely leveling spot for ten long levels. Good luck completing that level band without dying and losing some of your progress toward freedom several times.
While.not every game has a zone that causes some players to swear they will never return, they all have regions we'd as soon forget. What spots in your favorite game do you try to avoid? Did they feature really bad design, annoying enemies, or confusing layout? Was it an area devoted to an awful grind or series of truly obnoxious events? Or were they just spots soured by so many bad experiences that you can't separate them from the aggravation?