The answer is as easy as it is counter-intuitive: Tumblr is a New York company and Posterous is a Silicon Valley company.

Or, to put it another way: Posterous is an engineered product, while Tumblr is a designed product.

Posterous is extremely well engineered. There’s nothing wrong with it. Every single thing about it is well thought out. But it’s not just that it’s less pretty (though it is). It’s just not designed as well as Tumblr is.