I thought they went a bit too fast to the "well, everyone's a zombie now, I guess" stage of the movie. It felt like there should have been more build up to that, although it wasn't missing entirely, there's some room there for them to explore a bit more the steady breakdown as people slowly begin to go insane from the infection/virus/comet.
Edit: Actually, in thinking about it, I guess my complaint would be not so much that it moved into that stage too quickly, but that it moved into that stage at all. The movie's not about zombies, necessarily, but about ordinary people breaking down and committing unspeakable acts that they would never think of committing in their right mind. But the second half of the movie becomes way more like a standard zombie movie than it felt it should have. What I'm saying is that it seemed like they were going one way with the second half, but then abandoned that and went the safer route in the second half instead.
It's not a huge issue, but it's a bit disappointing to me.