i would go as far as to say it's the new wordpress. it's incredibly simple and easy to use. they should make the transition easy though (batch import wordpress articles).
Posterous is what a startup should be: simple enough that it really needs no hype to promote it. To wit: it's not even the new "currency" of anything! :)
TwitPic is quite annoying with the banner ad that displays almost as prominently as the photos.
Someone should build twitter using the tumblr API.