You made me think, "If only there were a host where you could install the software with one click, and it would automatically update itself..." Follow that thread far enough and the line between a blogging service and a hosting service begins to blur.

The real objective, I think, isn't to get everyone hosting their own stuff, but to popularize technologies that both allow data portability (you can move to another service), and are easy to use.

But WordPress has kind of already done this. They have step-by-step directions[1] for moving from a WordPress.com blog to a self-hosted install.

People who prefer Tumblr (an example of a blog host without an export function) to WordPress do so for reasons that seem orthogonal to data portability:

1. simpler, even easier to use,

2. aesthetics (sleeker design),

3. social networking features built in,

4. reach (because of #3, it's a lot easier to accumulate readers and engagement).

In theory, a service with an open-source codebase, or support for a standard export format, could provide all this. In practice, one hasn't.

Edit: icebraining pointed out downthread that you can mostly export a Tumblr blog by adding '/rss' to the URL of each page, a process that is easily automated.

[1] http://en.support.wordpress.com/moving-a-blog/#moving-to-wor...