I found a post where the user ask a valid question, then answers it in their comment. The question is now left open. What should we (as moderators) do?
When I find situations like these, I post a comment to alert the user to create a real answer and mark it as accepted once the allotted time has passed. It also wouldn't hurt to explain why it should be an answer instead of a comment (benefits to the site, future visitors, etc).
I find that most of the time, letting the user know to answer/mark accepted that they will follow through, assuming they come back to the site.
In general it is better to educate your community so they know to leave answers as answers.
An answer left as a comment is fair game to be expanded upon as someone else's proper answer, too.
In extreme cases, developers can copy the comment to an answer and then reassign ownership of the answer to the question asker.
This is a slightly different case, the user had answered in the question as an edit. Comments had been left to put it as an answer but as it hadn't been touched for a while I took the answer out of the question and posted it as an answer. Made it CW as I can't reassign ownership.
Educating users is good but if they are occasional, or one off users, leaving the question in a good state will help educate future users and benefit the site as a whole.
If the user doesn't write what he wrote in a comment as answer, or another user doesn't answer to the question, then the question can be voted to be closed as too localized.
A question without up-voted answers are considered unanswered questions, and the number of the answered questions is a parameter used to judge a site that is still in beta phase, like Drupal Answers.
Questions that have been closed are counted as answered questions.