I know the tip is to refrain from giving an answer in a comment, but I've found that in "unanswered questions" and elsewhere, there are a few like this. Is there a way to change someone's comments into an answer? In the feed it appears to be an unanswered question and may deter someone who needs help from reading into it.... What do ya recommend doing when this happens? Is there a way to add a feature to turn a comment to an answer?
You can't turn someone else's comment into an answer from that person, which is probably a good thing -- if the person wanted to answer, with the rep gains or losses that come from that, he would have. But if you see useful information in comments that you can use to write an answer yourself, go for it! Usually a comment doesn't contain a full answer anyway (too short), so you'll need to do some work to flesh it out. If the person who shared the idea didn't care to do that work, there's no reason you can't do it.
In very rare cases perhaps a comment is a complete answer. In that case, I'd first try pinging the author of the comment to suggest he turn it into an answer. If he doesn't, I might consider copying it, with attribution, into an answer and marking that answer Community Wiki. That way there's no appearance of seeking gain (reputation) from someone else's work, and the entire community is invited to edit to further improve it. (Because really, an answer that fits in a comment probably could do with some more filling-out.)