Like Twitter, Tumblr has some socialness that makes it quite easy to have a conversation larger than yourself. Visible Liking, Reblogging, and Following are implemented in really nice ways. Wordpress and Posterous are great, but you're pretty much by yourself. (Posterous has some basic socialness, but not like Tumblr) With Tumblr it's easy to discover stuff you're interested in, and have people find your stuff w/out having to rely on hustling your work. From the outside it's hard to tell (and just looks like a blog) but from the inside view of a user, it's really quite like Twitter not in format, but in experience.*

* I make many comparisons to Twitter here. They are extremely different. But they are both social and function like places, so I reference Twitter to make it easy for outsiders to grok.