I built a prototype of something like you describe, and felt that it really helped keep myself accountable. Happy to add you as a test user when I re-deploy it! Email is in my profile.
I use Pelican, but a lot of people like Jekyll, too. Pelican has good documentation, and I like that it uses a makefile because I was able to painlessly add in my own deployment command that pushes to S3 and an option for compressing CSS and JS.
I'm curious about the pricing situation on Elastic Beanstalk. T1.micro instances are very affordable, but is it wise to run a webapp and Postgres on the same instance? I'm not expecting a large audience, but I've heard that one should generally have separate servers for the two.
Perhaps creating a command line interface for Tumblr / Posterous hosting would work. I wouldn't try to reinvent the hosting platform though, if that's what you're asking.
Hosting one's own blog is trivial, and I'm willing to bet that your target audience is more likely to use a static site generator like Jekyll or Pelican.
Is there a tool that will remove the selectors for you? It would be nice to have something included in a site build process that does this for particularly large CSS frameworks like Bootstrap and Foundation.
If you're unhappy with the default Bootstrap look, http://bootswatch.com/ has nice free themes. It made a world of a difference when I ditched the default and used one of their themes on a current project.