This is a question which started off with a comment, then turned into an answer, and now it's a question in its own right (as suggested by Jeff). I'm not sure where it could go from here :)
It's obvious that different people cast downvotes for different reasons. (Upvotes are less controversial, generally.) Here are some possible reasons:
- Answer is wrong
- Answer is misleading (may be technically accurate, but will lead to readers making mistakes)
- Answer doesn't match the question (e.g. a C# answer to a question about Java)
- Answer doesn't really address the question, e.g. suggesting a completely different solution, even if the question specifies that there are good reasons why the current approach has to be followed
- Answer is okay, but not as good as another one which currently has fewer votes
- Answer appears to be plagiarised from existing answers.
- Answer is by someone I don't like.
- Answer is abusive (profanity, spam etc).
- No reason, I just felt like it.
- I'm a Happy Vertical People Transporter, and prefer down to up. Have you considered down?
- Ideological grounds: answer suggests a technology I don't like, or expresses a negative opinion of a technology I do like
Personally I go with 1, 2 and 8 - although I'll add comments suggesting changes in various other situations.
Have I missed anything? What do you do, and why? What are the net benefits (to you, the world in general, and the answerer) from your approach?