The tiered storage stuff is pretty cool. You can say "I want this data on this disk, so if I get a cache miss from a faster disk/RAM it'll come from this other disk first."
I believe it also has some interesting ways of handling redundancy like erasure coding, and I thiiiink it does some kind of byte-level deduplication? I don't know if that's implemented or is even still planned, but I remember being quite excited for it. It was supposed to be dedupe without all of the hideous drawbacks that things like ZFS dedupe have.
EDIT: deduplication is absolutely not a thing yet. I don't know if it's still on the roadmap.