I didn't know, RegualarKarate, that when you said "basic" you were referring solely to character development. So now then...
It seems to me that you over-simplified the character of David Sumner -- in order to bolster your argument -- by describing him as "a pushover who gets pushed too far and then fights back." Really, though, you can do that with any movie. Hey, let me try...
Warning if you haven't seen Double Indemnity, don't read on, as there is a SPOILER SPOILER SPOILER SPOILER SPOILER SPOILER SPOILER SPOILER
Double Indemnity: Lonely guy falls for a girl and gets cajoled into murder
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Sounds pretty basic, huh...and yet, Woody Allen called it the best film ever made. The way I described Walter Neff, much like the way you described David Sumner, made the character sound wooden and two-dimensional. It's all about the details, though. In fact, Straw Dogs is a character study about an American intellectual living in another country. His marriage is hollow and lifeless, but he doesn't want to admit it to himself. He constantly tries to ignore his emotions -- namely his anger, rage, violence, whatever you want to call it. We don't learn much about his past , so it could be that he has had a violent past and is trying not to walk down that same road. Now if that isn't an inner conflict I don't know what is.
It's fine if you didn't like the movie. I'm not saying you're wrong or you didn't get it or whatever. People like things for different reasons and it's all subjective anyway.