pvagner- pageTitle is required. I would say in order to be compatible with twitter and other similar services it should be optional. Now if I publish a repost or like via micropub, bookmark plugin fetches a title from the URL. Twitter plugin then does a new style retweet also adding a comment.
pvagner* Reposts should really be a reposts i.e. there should be embedded hCite with the content source feed allows me to grab. Let's imagine that I will repost a post from facebook. I might be able to display it but other people looking at my site will just see some warning from facebook as their browser tries to embed that post for them.