Teachers on average don't get paid a lot of money from my personal experience with them. To me the biggest complaint I hear is class size. Keeping an eye on 40+ little bahstids every class is a bit much for even the best of teachers.
The problem with teacher evaluations is what people have brought up, namely the quality of students. In public schools if you get indifferent kids who only occasionally show up for class and parents who can't be bothered, why should you get sh !t canned because of that? Its not your job to parent the kids.
I'm all for teacher evaluations, but I just haven't seen a good system for ranking them over time.