New users often mention that they can't comment. Veteran users often say "you can always comment on your own question." I'm wondering about the disconnect, could it be that the new users are trying to comment on answers?
If it is I'm wondering if there could be an automatic message that tells low reputation users that they can comment on their own posts – maybe in response to clicking on the comment links. It could also mention that have a reputation on other SE sites carries over.