(Edit: How is this wrong? Was it my playful nickname for Twitter and Facebook collectively?)
Regardless, I think Posterous boasts simplicity over WordPress.
A standard WordPress install starts you off with 7 different active widgets. There are thousands of WordPress plugins to do thousands of things.
Posterous is an example of "less is more", in my opinion.
I'm much more likely to read a Posterous blog(with standard themeing) all the way to the end than I am most WordPress blogs.
Nearly every avenue of a default feature now found in tumblr has come from innovation that users have made use of when the site was still young and lacked considerable features.
Plus the easy reposting to so many other services. I really like that Posterous can repost to not just Twitter, but Identi.ca, FriendFeed, and Jaiku. And Wordpress, Blogger, Tumblr, and every other blog site. And Youtube, Vimeo, Blip.tv, Flickr and every other media site.
Etc. etc... The autopost feature + post by email is the dog's bollocks.
Read http://posterous.com/faq for details on what you can do with an email post on Posterous.