Honestly, I'd do away with the whole rep system. In my experiences, rep systems tend to make a lot of folks shy away from participating on a forum and posting their opinions. Meaning less traffic then that website should be getting. It also means more lurking and less posting.
It may result in more posts but how many of those posts would be nothing more than "I agree" or "good point" or something similar that don't add to the discussion. By clicking I agree I can tell the poster I agree or I like his post and I don't have to reply. If I have something to add then I can make my own post.
I do agree that too many negative options may cause posters to think twice about posting, especially new members. The disagree option is fine but unlike the agree or like buttons I think a disagree vote does warrant a post to tell why you disagree. That's usually not a problem though.