The reason many people dislike this Movie:
Expectations!
Now first of all, let me tell you that I love this movie. I have watched it on multiple occasions and always enjoy it. I watch a loooooooot of Horror Movies and i'm usually disappointed about how bland and uninspired they are. Especially in the Monster/Murderer Teenie setting where one person after the other gets picked off and everyone is too stupid to make a single intelligent decision (The other overused Horror Movie genre is the haunted house / family but I won't get into that here). I was so happy to see a movie that addresses all of these bad movie tropes. They make fun of these tropes while still having them in the movie.... but with an original explanation!
When talking to people or reading people's opinions on the internet, the people who disliked the movie usually have one of these two complaints:
1: It was a rip-off of other horror movies. Now this argument I don't see / hear that often but I'll go out on a limb and say that they didn't understand it was both satire and paying homage to other horror films. Maybe they don't like satire or just saw too many similarities to Evil Dead.
2: It wasn't scary and / or the ending was just too over the top / stupid:
Now this I see as a more legitimate complaint.... and it might be the fault of the trailer, which clearly advertised another exchangeable horror teen flick. Imagine seeing the trailer of Lord of the Rings and then finding out in the movie that it's a romantic comedy. If you're expecting something scary but instead get a surreal comedy satire with only elements of horror then it's absolutely understandable you would feel tricked or cheated.
When peoples expectations are not met they can often be really harsh towards the material, regardless of the quality.
Of course there are other complaints and nitpicks but in general I think the movie messed with people's expectations which is always a gamble.
Well that's my two cents but please feel free to argue / correct me if you think that I'm wrong.