Pick just about any MMORPG (WoW included). Warhammer Online is probably the worst. There are enemies conveniently placed throughout the entire path that you need to get to the real boss. Each is spaced just close enough that approaching one (for melee) triggers the one behind it. Of course, you're character only has melee abilities, so you don't have a choice but to trigger it. Cue the clearing of the room as one mob chains to the next. Finally, you get to the boss, finish him up (easier than the room before him would have suggessted), and find out the entire room behind you has already respawned.
I think that's the biggest reason I can't stand most MMOs.
I think that's the biggest reason I can't stand most MMOs.