Just out of idle curiosity...
Are you more likely to rate a message if you see someone else has rated it?
Are you more likely to rate messages from certain people or messages that have already been rated by certain people?
There are certainly some people more likely to receive ratings than others. I'd say "for my part", but that just translates to "in general" anyway, given half the board's ratings are mine
Some are just because I'm always glad to see them post because their posts are far too rare for my taste (Grytpipe, or Sparhawk, or BErt, or Hylian, just to name some by definitely not all). Others may be (very) frequent posters, but have a backlog of ratings I feel I want to give. PatrThom is an example - he could post a LMGTFY link and I'd still rate it "informative". Kags could write "Booh!" and she'd get a "Love it". Slight exaggeration, but there are posts of theirs which I'd rate two or three times, but I can't, so a post that might just fall short of qualifying for a rating from someone else will get the "benefit" of their name and thus an extra rating.
Other people's ratings don't affect whether or not I'd rate a post, but they can affect what rating I give - sometimes I'm in doubt between, say, "funny" and "the Thing", or "funny" and "pun", or whatever. If there are already other ratings I tend to give the one the post hasn't received yet.
I use negative ratings fairly rarely in seriousness; usually in jest. If I disagree, I tend to either ignore it, or reply with what and why. Heck, I think I've given Charlie only 5 "disagrees" in all!