1: The Music
Charles Bernstein knocked it out of the park. From the main theme, that could be best described as "playfully sinister", to the catchy end credits song "Too Bad You're Crazy", the soundtrack perfectly compliments the tone of the movie.
2: The Tone
The movie has a lot of lighthearted fun with the cast, pranks and the slasher movie tropes, but is also willing to get surprisingly dark when it needs to be. It never gets bogged down with over the top violence and perfectly walks the tightrope between horror and comedy, without sacrificing the scares for the laughs and vice versa. It plays with your expectations and, like jack in the box, it springs you along until it jumps out to scare you silly and make you laugh.
3: The Jokes
The cast is extremely charming and instantly likeable (lots of eye candy too!), but let them loose in a house full of silly pranks and practical jokes and you are practically guaranteed to win your audience over. It's all about the ways all characters get pranked / tortured and their reactions to the increasingly uncomfortable situations.
4: The Twist & Cut Ending
I mean the whole movie is about people playing jokes on each other so the twist is pretty obvious from the get go. The whole thing still works wonderfully right until the very last shot of the movie, but it almost makes you wonder what the movie would be like with the original ending.
5: Amy Steel
After stealing the show in Friday the 13th Part 2, Amy returns as yet another smart, resourceful final girl who is more than capable to handle the situation at hand. Amy Steel has incredible screen presence and it makes you wish she made more horror movies!