Ok, my answer to the complaints I've heard:
1. We don't know anything about the creatures?
Doesn't matter, the movie is not about then, it's about the family. Not knowing the much about the creatures only intensifies the suspense. We, like the Abbot's, are trapped in this terrifying world with them.
2. We don't know how it ends?
Yes we do. Krasanski's character gives his family everything they need to survive. The film is about a father taking care of his family and he does that in this film. Personally, I was glad this film didn't force feed me a cliched Hollywood ending but gave me enough space to sit and think about it.
One of the best films I've seen in a long time.