In a previous post complaining about Tumblr, one of my major points was that Tumblr does not have a comment system. I am obligated to correct myself.
Tumblr has a few different comment systems, all of which are limited. As far as I'm aware, there are also questions, replies, and third party comment systems. Questions and replies are so constrained that I'm not sure they deserve to be called comment systems. But I did manage to install a third party comment system, Disqus. The comments work fine, though I don't like that there's no rss feed for me to track comments.
(BTW, I could also install Disqus on this blog, but I probably won't unless anyone gives me a compelling reason for it.)
However, I do not think this calls for a revision of my original conclusion; tumblr is still terrible. The fact of the matter is that most tumblrs do not bother with third party comment systems. Reblogging is still the main method of discussion.