Ok, here's my take on this: Confessions of an Upvoter
As a kind of disclaimer, I'd like to stress that at this level I consider myself a decent "TeXnician", nothing more. Since most of the answers posted up to the time of typing this come from really experienced users I am looking up to, I thought an answer from a less experienced user could be fun. More so, because I happen to have an Electorate badge.
As other users pointed it out, TikZ questions will always get more votes for the fun factor involved, especially the Christmas tree kind. I must confess, I also tend to upvote them, although to a lesser extent lately.
Recently my participation has somewhat slackened, I am bogged down with other tasks for the coming months, but I still read questions on a daily basis and also try to find an answer (where no
@s are required
;)). Or at least check the MWE to see if I can reproduce and trace the error, especially if I think it's non-trivial. If I already took the pains of copy-pasting the code and compiling it, I upvote the question. I see it as an encouragement for new users to ask away, a question with 0 upvotes can be quite a turn-off.
Now that I think of it, I often vote to close duplicates. Sometimes it is just a matter of doing a more thorough search, but at other times the link to a previous question is less obvious. I gladly vote to close questions that are blatantly of RTFM type. I feel that if we don't curb them, we tacitly encourage them, which is in no way constructive. (And I get slightly annoyed by users posting answers when a comment already suggests it's been answered and 3 or 4 closing votes confirm that...) Nevertheless, I very seldom downvote anything. It needs to be serious business for me to do so.
I upvote mainly on answers that either are very simple and elegant or from which I learned something new: a new feature or a new way of addressing an issue. Also, I like detailed answers with good explanations about how and why the code posted actually works. I have learned a lot from such answers and they definitely worth being rewarded. Good "competing" answers are definitely part of this category. And I upvote sensible comments too.
And there are some other factors involved in my voting process, with which some of the users may not (fully) agree. For instance, I tend to upvote a question/answer with 9 votes, to secure a badge for the user. At the point where it earned 9 votes, I think it also deserves a 10th one. I know there are lots of users who avidly collect badges and nobody can deny the fun factor involved. (In the early days I was one of those hunters myself.
:)) And this leads to the second (somewhat controversial) reason for voting: I practically was preying on interesting questions (and answers) to upvote when I had some 50 votes missing to get the Electorate badge... All right - now I said it all.