It's not covered in 62 "What are comments? How do I enable/disable commenting?"
While I don't think that FAQ should be made longer (it's approaching tl;dr status), for the 'new' user, I feel it's a valid question and should perhaps have it's own FAQ. It could also include something about not being able to post comments due to being non-validated (which I know is mentioned elsewhere, but... yeah).
So, uh, comments?