Ruined by incompetent villains
*SPOILERITOS*
It starts of well, slow build-up, great atmosphere and all that... and then we're introduced to villains who appear threatening at first, but then we see them in action. And this makes the movie much less scary, especially towards the end. You know how often slasher killers are unstoppable even when faced with an armed victim? Well in this movie one of the killers is armed with a handgun and yet incapable of killing a guy slowly walking towards him with a baseball bat. Another one, armed with a pitchfork, gets severely injured by a girl wielding a key and then is killed by his own trap. The scary witch gets pulled down a shaft alien-style by one of the protagonists as if she was the victim.
Honestly, the villains in this movie were as scary as monsters in Doom. Yeah, they looked scary, but seeing protagonists kick their asses like that it was hard to take them seriously.
And then the ending, trust me, it seriously made me LOL.
So, you know the cliche slasher ending where the protagonist escapes, thinks he/she's safe and then the movie ends by showing us that it's not over yet? Well, in this one the killer breakes into her house but tough luck for him because our heroine watched Home Alone as a kid so he gets greeted with a series of childish booby traps and gets his head blown with a shotgun a few seconds after kicking the front door open. The end.