Okay, guys. Here's the thing I think most people are missing that no one who has actually seen the movie has seemed to point out:
This isn't a monster movie. It's a movie about a group of five friends that just happens to have a monster in it. It doesn't tell the backstory of the monster, the history of the monters, what the monster is or any of that (though if you followed the viral marketing you can pretty much put it together), because it's NOT NECESSARY TO THE MOVIE. The movie isn't about the monster, which is why we don't learn that much about it. What we DO learn about is the people in the movie, their story, and - in that vein, the movie is told and ends perfectly.
Yes, there is a lot of action, a lot of monster-destroying-stuff action, people blowing up, some HOLYSHITRIGHATMYFACE moments, etc. But the reason we are left with questions about the monster is NOT the same reason as why Lost always leaves people with questions (For the record I HATE Lost with a passion), it's because it's not necessary to the true story.