Moosh's Ways. There is a special spot in Hell for Moosh after he created that abomination.
Going a bit more general, I can think of lots of things.
- Mandatory kill Peahats rooms.
- Bad 2.10-style custom bosses (there can be good ones, but the majority I see are bad).
- Unmarked and unhinted at bombable or walk-through walls.
- Death Knights, Patra 3s, or any similar enemy thrown down without any thought as to how the fight is supposed to be fair.
- Bland and empty overworlds.
- Forced grinding.
- The ladder (bad).
- The ladder used with 8-way movement (worse).
- The ladder and the raft used with 8-way movement (why Evan, why?).
- "Joke" quests that make no effort whatsoever to be good.
- Unmarked, unhinted at, and basically unfindable secrets.
I'm sure I'm forgetting a lot, but that's what comes to mind at the moment.