I am copying most of my answer to a Meta SE question about Can we allow moderators to upgrade a comment to an answer?
I would encourage all users who see "comments that answer questions" not finding their way into an answer to write an answer that:
- starts by saying "As commented by @...:"
- quotes the relevant comment(s)
- adds any extra value that they can to the answer
I normally only do this after a few days but I would have no issue with it being done sooner than that because we want to encourage "answers in answers" not "answers in comments".
If a user is worried about getting reputation points for what is effectively someone else's incomplete work to answer then they can make such answers Community Wiki.
However, I know that if I were putting my heart into writing comments, and then later noticing that someone else was getting points for taking the time to complete my answers, then I would be much more likely to write an answer rather than just leaving a comment next time.