I've added a brief subtext based in part on your suggestions. Let me know what you think.
Edit: Actually, I think if you said something like "Sandstorm is an open source app platform for personal servers" that would be a major improvement. The whole "app" part is missing from the main tagline. Then, your sub-tagline goes into more detail about what apps.
Edit 2: Actually, I would remove the open source part altogether. It's redundant if you have a github link somewhere on your page, which you do, and I think the developer community you are targeting would assume that it's open source. Or, just keep "open".
Thanks for the feedback, though! We'll think about inserting "app" in there.