The silly "fatique" system.

The slow, slow turn-based that I cannot stand.

The bizarrely-paced combat when real-time is turned on.

The idiotic NPCs that join your party.

The lack of balance between magic/tech and magic/melee characters.

Yup, Arcanum ranks as one of the worst games I've ever played. Even Dungeon Siege is better, because it's at least playable and you can control every characters, so no need to suffer moronic AI.