I can comment on my own answers and questions, but not others.
Is there some reputation rule that I'm not aware of, or is this a bug?
Meta Stack Exchange is a question and answer site for meta-discussion of the Stack Exchange family of Q&A websites. It only takes a minute to sign up.Sign up to join this community
You can always comment on your own questions and answers, but you need at least 50 rep to comment on other peoples' posts:
Amass enough reputation points and Stack Overflow will allow you to go beyond simply asking and answering questions:
50 Leave comments†
† you can always comment on your questions and answers, and any answers to questions you've asked, even with 1 rep.
As kyle noted, you need 50 rep to do so... Check out the FAQ for StackOverflow to see the other reputation restrictions, etc.