So, it would be more a micro-blogging platform than a blogging platform, right? Because I find hard to go with the micro-blogging route when you have characters limit (it's just an exemple but there are surely some restrictions). I mean a micro-blogging platform it's good for short discussions mainly.
The audience matters much than the content, how are you going to "monitor" the users. The existing ones and the new ones. Letting other users to "vote" for an exclusion? The system could end up with interests conflict.

(I want to be the first to post: "who is Satoshi")