The difference, of course, being that when the original authors (or the maintainers of the original metaserver) go to the dark side, it's not hard for a nonprofit benevolent geek to open up her own server, and allow users to migrate their data over to the new server and remain on the same network, keeping all their connections intact. At that point you're basically moving your website (your social network "node") to a new host (another Diaspora server), while remaining on the same network!
Right now, if you're fed up with how Facebook is handling your data, you have to quit Facebook. With Diaspora, if you're fed up with how your Diaspora server is handling your data, you switch servers, or even start your own.
Furthermore, the whole thing is going to be free software, so in the event of the original authors trying to slip nasty stuff into the code, a) we can know about it and b) it won't be hard to fork the project into a compatible non-evil project.
This is radically different from Facebook's philosophy of "we own all your data and you're stuck with us." My only hope is that the Diaspora guys can make their stuff easy to use.