This is a very bad idea, because it is often very subjective as to what is a comment or an answer. And what would happen to the votes (up or down) would they be refunded? If not, then you're saying that the people who downvoted can never get their rep back.
Also, a user may have put quite a bit of thought into their answer which you think is a comment.
Edit in response to comment
Marking something inappropriate is less subjective, because you are offended by it. It not a contextual thing, it's something visceral. When you are marking something as offensive, you are saying you want it expunged from the site, which is a pretty big deal, although necessary at times.
Yes, it is also subjective as to whether or not something is the 'right' answer, especially when two things both helped you out. In those cases, all you can do is upvote them, and mark the one which is most helpful to you, again it is okay that it is subjective, because it was asked by a single person, and the selection of the 'most helpful answer to me' should also be made by me.
However, marking something as a comment immediately precludes them from receiving any votes, it also has the potential to steal votes away from them. And, as I mentioned previously, some answers ARE comments. For instance:
Question:"How can I best expose my website to
hackers? I really want to, because of
XYZ" (not a real question)
Answer: "XSS can be exposed to by this
Answer: "Exposing your site to hackers
can have no benefit, it will be a
danger to your hardware, software, and
all your clients, and any benefits
will be BY FAR outweighed by the
destruction that would be wrought."
I'd say it is obvious that the second answer would be correct (in this very contrived question), even though strictly speaking it is a comment on the question. The second person sought to go to the heart of the question, which means that rather than answering, they were clarifying and commenting. This means that the second answer should be moved as a comment, even though we can easily recognize that it is more correct than any 'answer' could be.
Second, if you'll grant that there are situations where a 'comment' can live on as an answer, it means that you are creating a gradation of 'good comment answers' which can stay and 'bad comment answers' which must go. And generally, the measure is going to be whether or not they are useful. If they aren't useful, they should not be posted at all, they should be downvoted, and a comment should be added to say 'this is not helpful, nor an answer'.
In any case, moving the answers away will be troubling and frustrating to the person who posted what they thought was an answer. Furthermore, you are taking away the ability of the community to up/down vote them, which is the entire purpose of the up/down vote buttons, it is to encourage and discourage certain bvehaviours. When you move it to a comment, we can no longer do that.