I'm sorry if this has been suggested before but it seems to be a common occurrence that people answering questions (at least on Stack Overflow) post their answers as comments (perhaps to avoid getting downvoted). If said comment happens to be an acceptable answer, the asker of the question asks the answerer to repost their comment as an answer. Wouldn't it make sense to allow the asker/answerer to be able to 'upgrade' their comment to an answer so it can be marked as an answer or voted on as such?
In the end I think it would help future searchers find their answers. Personally I generally ignore comments because if it isn't an answer I don't consider it to be important. Perhaps this is just caused by answerers not following best practices of the Stack Exchange family of websites but I still think that this feature would be valuable.