It never explained why Donald hated Billy so much, or how he got burned. I kept waiting to see some flashback or monologue to explain it, but it never happened, so there was no pay off for my patience there. This made him such a one dimensional villain. There was that dream sequence where billy flicked the cigarette at him, but that was nowhere near good enough.
I don't think the other characters were fleshed out enough either. I didn't really care about any of them, like the daughter was complaining about how crazy the case was making her mother but that wasn't really conveyed since she seemed fine when she talked about the case in the law firm, and she wasn't even really working on it, she just chipped in once in a while with some commentary.
And what happened with the prostitute and the cop at the end? He pulled her over and pushed her down but then we don't find out what happened there. And she got really screwed at the end, after all the work she put in helping Billy he totally blanked her.
I didn't get why they killed that Carl Stoltz guy either. I thought he was going to be a "mini-boss" since he was doing all the dirty work and then he just ended up getting murdered off screen with no explanation. I didn't get the relevance of the cop telling the prostitute that (forgot her name, something "Gold"?) either. If anything, the copy should have been killed for giving up Carl's name and being a loose end since he beat up Billy and gave him the DUI charge.
I don't think they did they explained the psycho lawyer falling in love with Donald well either. She had never even met the guy before and then from the very first time she sees him she's suddenly in love with him even though he's over twice her age and hideously scarred. There wasn't a proper build up there IMO, and at the end when she sees he was using her, she still loves him! Made zero sense.