I recently came across this question on apple.stackexchange.com and wanted to add a comment, not an answer. But I couldn't since I didn't have the necessary reputation. But I could post an answer. I understand the reasoning for that.
However, what I found frustrating is that since I am not a heavy user of any of the stackexchange sites, I didn't realize that I need more points to comment so I found myself searching for the "add comment" link. I looked all around, couldn't find one, and since I thought it was best to provide feedback even though it wasn't an answer, I posted my feedback as an answer. Later, someone else moved it to a comment (which I obviously agree with).
I later had to search around to find out that I needed more points to comment, which a less motivated user wouldn't necessarily do, and decided to post here (I hope this is the correct venue for this discussion)
My point is that I know I can't vote up an answer unless I have over 15 points because the site tells me so when I try to upvote. Somewhat analogously, I think there should be some sort of hint or pointer about one's inability to comment on a question's page from a usability standpoint. My guess is that would reduce the number of times answers are converted to comments as well. It might even increase a user's desire to answer a different question so that s/he would have enough points to comment elsewhere.