I hope you've seen 'District 9' because I may mention it a lot.
'Monsters' is the exact opposite of 'District 9' in my opinion both films are supposed to be a social commentary, they set up a huge world with the history of the aliens already ingrained in and then go "ha ha looks like we're the real monsters here". District 9 did it well, it showed you the miles and miles of filthy, infested slums, it showed the consistent racism of every human character toward the prawns. Monsters had a couple of wrecked tanks and boats and said "no totally guys, there was a big alien war".
The characters in District 9 were generally well-rounded and even flat characters were still recognisable and interesting. In Monsters the characters didn't do much of anything, I was aware that they were both humans and one of them had a fiancée I think? but they were flat, boring and didn't really do more than stare slack-jawed at the three aliens you see throughout the whole movie while asking stupid questions.
And finally the aliens. In any movies like these, the whole point is to throw an outside influence into normal life and see what happens. This is then used to support whatever message the writer and director had in mind, or just to entertain the audience.
District 9 is the movie equivalent of a serious philosopher, it said "If aliens landed here tomorrow it would open new heights of discrimination and exploitation, we'd maim and kill the aliens and even our own kind just to get our hands on their resources and in the resulting clusterfuck the almost non-sentient alien worker ants seem more human than we do."
Monsters is a stoned high-school student who sort of knows what philosophy is, it said "Whoah, man, like can you imagine that if aliens landed tomorrow we'd have like wars and refugees and shit...? Doesn't that... doesn't that blow your fuckin' mind?"
Basically I'm pissed, because while District 9 was thought-provoking and interesting it still had time for comedy, several gunfights and an alien battle-mech, while Monsters... Well, Monsters barely even had aliens in it...