“The question has arisen; is there room for something in the middle?” - Steve Jobs, on the announcement of the iPad.
Along those lines:
Tumblr needs comments.
DISQUS has done all of the work for you with points/stats, reply notifications by e-mail to foster continued dialogue, most popular comments rising to the top of the thread, etc.
You can still have Likes and Reblogs, but there’s definitely room for something in the middle.