What a great demo! We're currently building a critical service on MongoDB and feel a bit leery about some of the issues others have expressed at scale. Would be interested in testing FoundationDB for our purpose in the near future.
You mention scale - do know that what you're seeing in the video is probably the weakest FoundationDB cluster ever assembled :) Our in-house testing cluster is a 48 machine, 192 core setup, and it works great and scales almost linearly. You can see our performance numbers at www.foundationdb.com/performance (though those results are outdated now, the tests were on a cluster half the size and several versions ago).
If you'd like to chat about your particular use case, send me an email at nick at our domain.