I have 19 points on SO and see that I can't leave comments on questions that I did not originate or answer. I thought it had something to do with my level, but then I noticed someone with less points leaving a comment. Am I missing something?
Without a link to the question you are referring to it's hard to say why that would happen, but from the SO FAQ:
As already mentioned this does not apply to our own questions and answers.
Reasons why a user may have left a comment and now has low reputation would be:
However at the time of leaving the comment they did meet the criteria.