It seems as though one is not able to comment on a shared item unless the owner has commented first. Is this intended/desirable? I can understand how this might not be desirable for some people, but a friend just shared a comic that I'd like to comment about and the option doesn't show up.
Yup, as designed. I remember writing out my thoughts in another thread, but essentially in order to comment you have to "own" the comment, which means you have to share it. Otherwise, you can reply to other people sharing it. But it would be confusing if you could comment but it wasn't a share. Just how it is.