I'm going to start off by saying that I am very familiar with the original play, "Bug", (originally written by the screenwriter, Tracy Letts); at one point I even performed a monologue from it. And I am a big fan of the script, so I do know what I'm talking about.
1. Ashely Judd. She mostly bothered me at the end, but the monologue she performed at the end, (with the infamous, "I AM THE SUPER BUG MOTHER" line) was really melodramatic, to the extent where she throws her hands up in the air and screams it giddily. Twice. Now, I realize that she's supposed to be portraying a mental breakdown, but, seriously, she could have brought it down just a touch.
2. I think that the camera is what killed a lot of it for me. The stage show is always superior to me because you can see everything; with the camera angles, you can't absorb the scenery as well, and not all of the characters are always visible, even if they're in the same room.
3. Another thing that I really liked about the stage version is that it stayed in one room--always. I hated that they actually went to the bar where Agnes and RC meet up for the night. I didn't like the flashback to the grocery store. Even leaving the hotel room, if just to go out and smoke, was a big let down for me. As I interpret it, the single room was a metaphor--the characters are trapped in their own delusions. By letting the characters leave the thing keeping them secluded, I don't get that sense of seclusion.
4. The way the room shook when the helicopter went over the motel. This was really just a minor thing, but I think the sound of the helicopter would have worked much better; it was a much more subtle thing, and the room shaking was a big annoyance--another example of the film going over the top when it works much better as a subtle descension into madness.
1. The script was written by Tracy Letts, who did write the original script. That was a big plus, because the lines were, for the most part, intact.
2. Michael Shannon. He was great in the role of Peter--he pulled off the quiet creepiness that I liked very much.
The film is very hit or miss, (but mostly miss). Go check out the stage production instead. It's a thing you need to experience live, and not on screen.