I've noticed that wordpress.com improved their commenting functionality with 2 useful things:
1) When "Users must be registered and logged in to comment" is active (which I want) visitors can actually log in using their WordPress.com, Twitter or Facebook account (there are 3 small icons below the comment textarea). I've seen several "social" or "Twitter" plugins on wordpress.org but am not sure which one does both Twitter and Facebook for comment forms.
2) There is an option "Notify me of follow-up comments via email" below the comment form and in the settings, there is the "Show a 'follow comments' option in the comment form" option. Again, which plugin does that?