The OGS already has the option for users to make a sgf library (it is in the menu on the left). I think it is a feature that is just not much used. Perhaps someone could compile nice collections and advertise them here in the forum.
About the commentaries, I didn’t understood how this would differ from the chat that already exists in the game rooms (except for the voting). I think it more a matter of usage. This kind of suggestion is appearing very often here, how to make the site more friendly to kitizing and how much this is a site issue and how much this is a community issue… no one really knows.
Cheers!