Posterous is just a blogging platform that lets you use existing technologies to author and publish posts (to various locations). I routinely publish hundreds of words to my Posterous blog, and nobody has reprimanded me thus far.
Oh well, people can use us however they want, compare us to whatever they see fit. When people say they use Tumblr, I ask them how they post 3 photos, and they are stumped :)
Could you please reveal a bit of your future plans, for example what will be in the paid version? ;-) Or at least, I'd be very grateful if you can tell whether you are planing to allow displaying ads in the future.
I use Posterous as my platform for publishing content that's text-based - I really like using my email client as a UI, and it makes it easy to post from anywhere. It's also dead simple (once it's set up) for non-technical users to run a blog using that interface, and would be of great use (I imagine) in a situation where bandwidth is at a premium, like in remote environments where all you've got is an expensive satellite uplink.
Tumblr, on the other hand, is very much more of a social/media blog platform. It's got that feature where you can re-blog something and it automatically links back to others who have done that, among other neat features. I'm also a big fan of its bookmarklet for sharing links and media, and while it's fine at doing text, it's in no way exceptional.
In short, I consider Posterous to be a very well-executed text-centered longer-form blogging system, whereas Tumblr is more optimzed for media/content sharing and short-form text - although both would work for either niche.