I've seen many useful questions, where someone took the time to write an excellent answer - but did obviously not give it an upvote.
On several sites I take part there exists the understanding that if a question is good enough to get a good answer it deserves an upvote.
(I was so very tempted to write an answer and not upvote, but annoyingly someone's already given it an upvote so my asinine joke would've been wasted ;) )
I feel like I upvote a lot of questions regardless of whether I write an answer, but I don't upvote every question I answer. I tend to upvote questions where:
a). I think it's likely to be useful to other people, or
b). I find it interesting, or
c). I get the impression that the asker has put considerable effort in, either attempting to solve the problem themselves, or spending the time to narrow down/explain what their issue actually is, or
d). I just get the nebulous feeling that the asker deserves an upvote.
I use completely different criteria for deciding whether to answer a question:
a). I know the answer, or
b). I don't know the answer, but it looks interesting/fun and I think I can figure it out, or
c). I just want to get it off the Unanswered Questions list.
The two don't always line up.
