15th

Posterous is a blogging platform. A blogging blatform where everything can be post by email. If only it was that, it would be great.
I like the way they succeed to simplify their sign-up process without even having one. You simply email them at post@posterous.com and your account is done.
I also like how they succeed to implement an easy, but hightly powerful structured process to email your medias and text to other social sites. For example you can email your pictures to flickr@posterous.com and they will be post to Flickr. This way you don’t need to remember all the private-easy-to-forget-emails (ie: adamDFR23@flickr.com) of each of your social media sites who accept email posts.
The problem in their platform is they try to be and do everything. It’s become difficult to understand what it is and how it can be useful: just the idea of being able to blog everything by email is an amazing feature. Why having added (and be forced to explain) all the other features? By extending your offering you may also loose your main focus and strength.

Tumblr for example, who offer mostly the same set of features (except trying to be an hub for your other social sites - and thank your for not trying) succeed without overwhelming their users and ever loosing their main focus: being the easiest blogging platform.