Tumblr is the most polished blogging platform out there. It also has built-in audience controls, meaning if you write good things, news will spread.
Posterous has an excellent feature set, but it's uglier than balls. Bad typography, bad colors, generally washed-out. If you're okay with not looking beautiful, though, it might be a better from-the-get-go choice than Tumblr. I've tried and given up on it myself.
Soup.io is another tumblelog platform that makes it dead simple to reblog (literally one click). http://soup.io