Tons of working features, a more well-defined roadmap, (much) more experience, and it's way easier to install (LAMP vs Rails). Also, Appleseed is a full social platform, that allows you to build components and extend it. Diaspora is more of a single application, it doesn't have any kind of modular framework to build off of. I don't think Diaspora will ever be dead in the water, but it just has so much catching up to do, and not small stuff, big architectural stuff that doesn't really benefit from the "many eyeballs" advantage of popular open source.