Before seeing this movie, I had a couple of guesses about what it was about: Blair Witch meets Godzilla (easy guess obviously, though they never clearly showed that there was a large creature in the previews), and that the monster was the head of the Statue of Liberty come to life (with the help of aliens and nanotechnology, or demons from hell).
In the featurettes there are numerous clips of the director asking the actors not to share anything with the world in order to keep the movie a secret, but there isn't really much to keep secret except for the monster designs. There is no explanation or big twists at the end to give away. But as for the monster design, the smaller creatures look sort of like the creatures from Tremors 2 (and Tremors is mentioned by a character when the party moves to the rooftop) or the bugs from Starship Troopers, or any number of video games- the larger creature I suppose is sort of original but still looks like a derivative creation that any art institute of whatever graduate would create.
The director makes a jab at the US made Godzilla movie from a few years ago by stating that he wanted to do an 'American Godzilla', but then the smaller creatures directly correlate to the velociraptor babies from that movie- the movie can't think of enough to do with itself with the big creature so throw something smaller in that is more easily killed. The invincibility of the big creature in Cloverfield makes it much less interesting- it could have survived the heavy bombing but should have been visibly damaged, or it could have survived more on wits by avoiding direct hits.
