Too much gore, or grotesqueness.
Completely or almost completely animal/whatever head on a body that is otherwise a mid-range mix of human and other form, I'm looking at you, 90% of werewolves and anthros. (Altered Beast gets a pass, because it's Altered Beast
https://www.youtube.com/watch?v=fzC7jzA1nwc ).
Forms that are extremely non-humanoid, loss of hands, arms etc, though sometimes some lower-body is acceptable, mermaids, nagas, drider etc. There are other exceptions, especially for comedic effect.
A lack of any mental changes. I always feel like the mind should be influenced to some degree by the transformation. There are exceptions.
Complete mental change with no trace of previous identity remaining. (Personality, memories, awareness of previous state, continuity of consciousness of self, if nothing of these remains the transformation seems pointless to me).