#4 is interesting—but how do you define "devices?"
I have, say, 500GB of music. It doesn't fit on my laptop, so it "lives" on an external HD. Could it sync the laptop with the HD so that I always still have access to a set chunk—say, 10GB—of my most-recently-used music files? This is a capability I've been waiting for something to support ever since I bought the drive.
a device is anything that can run AeroFS. We can (and plan) to do exactly what you described for caching data in between _devices_, but we haven't really considered the use case where you may want access to your most recently used external HDD data (in this case, the external HDD is really more like a part of your laptop device)