What surprised me the most was that I actually gave a shit about the characters. Drew Barrymore's Casey was probably the first ever character I felt sorry for in a slasher film. Usually these films just assemble an array of victims and wait for you to take joy in their butchery, but Barrymore makes you care for her character and the effect is utterly horrifying when she finally gets killed.
Same thing with the ancillary characters who all have these cute little moments which actually make you kind of like them. They're all potential victims, but for once I'm actually wanting some of them to live.