Journals such as AMTD and ACPD contain an open discussion where anybody can comment on papers under review. The peer-reviewers are still anonymous, but other people commenting are not. If I'm reading a paper and have questions about it, what are the pros and cons on posting them as public comments versus writing an e-mail to the author directly? On the one hand my comments may improve the paper; on the other hand, if I write something stupid or step on somebody's toe, that may harm my future career.
What do others think?
(Edit: I might add that in the case I have in mind, I am already in contact with the author)