I already know that enough people posted their comments as answers to the question for the option "not an answer":
This was posted as an answer, but it does not answer the question. It should possibly be an edit, a comment, another question, or deleted altogether.
But what about the other way around? I've seen several instances where a (usually simple) question is answered in a comment rather than an answer. What should be done with these comments, since the Flag button doesn't give a "not a comment" option...