Another feature I'd like to add is an ability to follow people, but I'm worried about the exact implementation since the current assumed contract is that your library is private.

One more feature of course I hear about often are comments, but I'm afraid of the site disintegrating into YouTube comments. I think comments have to be done very carefully and would require significantly higher code complexity to incorporate moderation tools, etc. Tricky and non-trivial not just implementation wise but design-wise, incentive-wise, etc.