You bring up two separate issues, and I think they need to be addressed separately. I agree that votes should be reversible to correct mistakes. Votes are, I think intended to be more of a quality filter than an expression of agreement. Changing your vote because you now agree with a comment is a Bad Thing.

Voting because you agree or disagree is a Bad Thing for a quality filter. Unfortunately, though, all these sites start off with a quality filter and end up with a demoralizing popularity contest. Voting should cost you something.

The quality of a comment is not orthogonal to its truth.

Nor is it identical. That's the problem.

I agree!

Well, just don't vote me up for it...

I agree to disagree.

This site has never made clear what the voting is for. I used to use it for strongly agree/disagree. Then it was changed because someone complained they'd been downvoted on all their previous posts so only up arrows exist on old posts. Since I read interesting looking articles that appear in RSS I only read old posts so this effectively barred me from participating. Dropping down arrows was a poor solution to the problem.

I then realised that the remaining up arrow could be used as a `I've read this post' indicator and started clicking all of them as I read old posts. Very useful to help pick up reading where I left off. Funnily enough, I'm not aware of anyone complaining that they'd got lots of up votes over the period I was doing it. :-)

Me too.

Once I realized that voting up was the best way to keep track of stuff I might want to come back to, I used it for that, even though that means voting up stuff only I would care about and not voting up stuff that might be useful to the larger audience.

Comment voting is a little more interesting. I vote up comments I think reflect an interesting point to the discussion, even if I disagree. But I know that lots of people vote comments up or down solely on a basis of whether the comment agrees with their pre-conceived notion of reality.

People actually use the votes?