Sometimes a comment will suffice to answer a person's question. This may be because they are only missing one small step in the argument, or because they can be pointed to a resource that answers their question fully.
In such cases, it could be a good idea to be able to accept a comment as the answer to the question, in the same way that answers can already be accepted. (It may not be appropriate to award 15 reputation points in this case, however).
The advantage here would be that questions answered by comments could then be removed from the 'unanswered questions' section, making it easier for other users to identify which questions still need to be resolved.
What are your thoughts on this?