Ass Pull: John Carpenter himself admits that while writing Halloween II, the idea of Laurie being Michael's sister came to him "at 2:00 in the morning in front of a typewriter with a six pack of beer."
Contested Sequel: Though it's certainly one of the better received sequels, some people felt that the twist that Michael is Laurie's brother downplays his character to a degree. However, a lot of Michael is still a complete mystery, so the twist is acceptable for most fans.
Squick: The kid with the razor blade stuck between his teeth, presumably from when he bit into an apple given to him by a so-called "Halloween Sadist."
Too Cool to Live: Dr. Loomis. That is, if you follow the H20 timeline instead of Jamie Lloyd one.
Villain Decay: The revelation that Laurie was Michael's sister. Some felt that the reveal took away the menacing mystery that made Michael such an effective villain as it gave a reason for him to stalk Laurie instead of leaving it as an unknown motive. Others felt that Michael's obsession with killing his family members may make him out to be one of the more brutal or irredeemable slashers as even some villains care for at least one family member. This is still a rather divided twist to this day, but for the most part, people seem to accept it as canon as it's used again in the remake.