It's interesting that those that don't like horror/comedy wouldn't like this film. I'm not a huge horror fan, and have little patience for the typical slasher films.
Unfortunately to get all the jokes here, you have to know horror pretty well, and if you're a huge horror fan and don't like when it's mocked then you won't think the jokes are funny.
Maybe not the "best" horror film ever, but at least the most comprehensive?
Fans of Joss Whedon aren't mindless drones like Justin Bieber or Twilight fans. Saying that he could do anything and they'd worship it shows you don't know Whedon material very well.
But this movie does resemble one big, world-ending episode of Buffy - at least in tone and topic. However Buffy would have definitely ended the stoner (but she'd cry first).