I haven't read through all the responses so pardon me if I've repeated what someone else has said.
I don't think it would be wise to have a dislike button. There are people who dislike other users and I believe it will give them an additional way to harrass/abuse them.
Bottom line, if a person doesn't like what another user has written, they don't "heart" the response. That's enough of a "dislike" in my mind. Flagging takes care of the problem posts.
But, if you feel it's something you want to try, then program it. You can always get rid of it if it doesn't meet your expectations.