Tumblr's reblogging design says a lot of its peculiarity.
I find it fun reading my Tumblr dashboard everyday, in an age where blog seems done and RSS in decline—established media and writers make it a norm, whereas the rest of us have moved on to "microblogging". I find truly insightful writings and excellent blogs on Tumblr. It has a light mix of social network, like the elements of following, but only a very light mix. It's largely anonymous, like a masquerade ball: Many of its default themes don't even show your own intro, and that is if you bother to fill in that info at all.
Then there's this no-comment thingy. Not that you can't do it, as there are definitely ways to extend, but it's just not there as a built-in. Instead, reblogging is made insanely easy. Almost too easy that it could have become a retweet soup that deluged much of the Twitter scene, but apparently it doesn't.
I'd say that reblogging to Tumblr is what comments have been to "traditional" blogging. By allowing comments, each blog entry becomes a forum and a point of attention on its own. Tumblr, on the other hand, makes easy adding your own words when reblogging. And this is important: Because it's you, the one who reblogs, that is the center of the game, and you get a voice of your own. At the same time the original author also knows your existence. It is unlike the entry-comment design, in which comment is relegated to a secondary, albeit supportive, role. I've followed a number of interesting tumblogs by following the reblogging history of an interesting post.