(mostly non-answer intended as a reply to Jon's long answer)
IMHO, it's neither desirable nor truly possible to enforce consistency in voting. Some people will upvote answers they like, answers they think are underrated (sympathy votes, etc), or merely answers by an author they feel deserves a reward for actions elsewhere. And downvote for a similarly wide variety of reasons. If comments are encouraged or enforced, we'll almost certainly see some change in behavior, but I'm not convinced it will be entirely... or even largely positive.
My primary rationale for this line of thinking is that some votes on SO are already not anonymous, and already require a reason to be given: close votes require both. I think this discrepancy can be justified by the much larger potential impact of voting to close a question vs. merely down-voting it, and the corresponding desire for such votes to be relatively rare. None the less, you don't have to look far to find... spirited discussion following the application of such votes. Indeed, I'd be surprised if the outcry over perceived-unjustified downvotes even comes close to matching that over perceived-unjustified close votes.
Downvotes, even when unjustified, are largely benign on SO: the site subtly discourages them via a small rep cost for the voter, and largely mitigates the rep damage for the author of down-voted posts by charging only 1/5 of the points granted by a corresponding upvote. If you're getting a single, stray downvote here and there, it might injure your pride, but won't actually have much of an impact on your standing among other users. (Serial down-voting is another issue, though partially addressed by existing scripts and heavily discussed in other posts here)
My personal feeling is that votes - up or down - have their primary value as hints to other readers: which questions are worth reading, which answers should I read first, next, at all. And I vote accordingly, doing what the site lets me to adjust the scores of answers to a given question to where they match my own personal opinion of what matters. I won't vote a decent answer to where it scores < 0, but I see no reason to leave it > 1 if a better answer scores lower, because if I were searching out an answer to the question, I'd want to read the most comprehensive answers first. I don't have a hard and fast rule on this though; sometimes I'll vote on every answer to a question, other times I'll only vote for one or two.
Going forward, I think it's more valuable to adjust the site to accommodate different voting patterns than to try adjusting each user's individual voting strategy. The latter is simply doomed to failure: no matter how much work we do to come up with workable guidelines, and how much evangelizing we might do trying to get users to use them, there'll always be new users who haven't bothered reading every FAQ and who'll still vote, or old users who just don't care.
Obligatory "answer" portion:
One possible solution to the problem of confused users with hurt pride might be that found on Slashdot: anonymous, pre-defined "reasons" for the down-vote. A user seeing their answer voted "Inaccurate", "Dangerous", or "Misleading" should take a good hard look at their answer, while another seeing his answer voted "Overrated" might simply shrug it off.