Question: Mine is simple. I up vote any answer I think will be of value to the OP and to others with a similar problem. If I think the answer will only be of use to the OP (this usually stems from the question being very localized), I will up vote the answer when the answer takes a broad view of the question and goes sufficiently beyond the local needs of the OP to have archival value. I always feel sad when such an answer is lost because the question gets closed.

To be fair to all who answer a question, my policy requires me to revisit a question whenever a new answer has been added to up vote again if another worthy answer has been added. I accept this responsibility.

I'm pretty sure some others have different policies. I don't claim mine is best. In fact, I suspect it might be too simplistic. I'd like to see other policies expressed. One might be so much better than mine that I will replace my current policy with it.

Do you have a different policy? If so please write it up as an answer.

### Edit

I already discussed my criterion for rating a question as good in [this answer](https://mathematica.meta.stackexchange.com/a/1308/3066) to Mr.Wizard's question. That set my ***question*** up-voting policy because a good question is worth an up vote.

Answer: 
I wasn't around when this was asked, but I feel I should also be putting in my take.

My upvoting criterion is capriciously arbitrary: if I "enjoyed" reading the answer, or learned something genuinely new, or it is an answer that I would have written myself (also counting the case where it was written in a far better style than I would've done), only then do I upvote the answer. It is the exact same voting criterion I use on other SE sites, and I like to think it has served me well.

My downvoting criterion for answers is way more strict: I will only downvote if an answer is actively wrong/harmful, and the answerer has proven uncooperative in nudges (through comments) to fix his/her answer.
I think of my voting policy as very conservative. Every vote must be justified (at least ideally). Nonetheless, there's a lot of good content on the site, so it's not hard for even a conservative voter to have cast a lot of votes in aggregate. My voting is also very subjective, although it has become less so over time (cf. the second bullet points in each category).

Let's presuppose that anything I upvote is coherent and correctly formed, at least intellectually. (I normally correct minor formatting or writing problems, if there are any, before voting. This is particularly easy to excuse for newcomers to the site who bring with them fresh ideas.)

A **question** gets my upvote if

* it asks about a interesting problem that I would either have to think moderately hard about in order to provide a good answer (which is also the best way to attract my answer), or (for more challenging problems, if I don't have time to answer, and where I have insufficient understanding of the subject to provide an answer of my own) where I expect I could learn something from the approaches taken by other people. Or,
* where a definitive answer immediately comes to mind, but the question has extraordinary value for the community (e.g. it raises a commonly encountered issue in a particularly clear and incisive way).

An **answer** gets my upvote when

* I learn something from it. I very rarely upvote an answer that doesn't teach me anything. Although this doesn't have to be especially profound--in performance tuning questions, when people collect others' answers together and benchmark the codes against each other, that also gets my upvote, because it provides the answer to a natural question and leads to a better understanding of the existing answers. Or,
* it is a technically impeccable answer from a new user, even if not particularly enlightening to me personally. I do this because everyone needs some encouragement and a certain amount of points in order to be able to use the site fully and feel as if they're a part of the community.

I try never to vote for answers simply on the grounds of correctness. A correct answer must also fulfil one of the other criteria in order to get my upvote.