I used to like movies like: "The Matrix" and "Bad Boys". But I've grown very tired of violent conflict portrayed as easy, fun, moral, and without risk. This is a movie where all the violence is fearsome, difficult, and wholly unpleasant. As it should be, anything else really is dishonest and evil.
This is one reason i think most people don't like this movie. There are two more. One is a somewhat complicated plot. There are about a dozen characters and each one have different motives and I think most people can handle no more than 4 motives. The other reason is that the "main characters" are not the heroes of the story. They set the story in motion and keep it together but they are not who the movie is about and to tell a story in such an unconventionally roundabout way is entirely confusing for some people.
So you may not like this movie if...
You are uncomfortable with violence being violent,
You can't keep track of the motivations of 8 separate characters,
You automatically believe that the story is about the characters played by the actors pictured largest on the box.
otherwise you may really like this movie because it's really well made in all its aspects.