I've never been much of a BSD guy - tend to run Linux at home - but I do admire Matt Dillon a lot. A co-worker mentioned this
to me, and it's just immensely cool.
Dillon is working on a journalling FS that allow "unlimited" (i.e. limited only by available journal space) undo. I love it. The potential for things like security (an intruder can delete whatever they want, and you'll just undo the changes and voila, you can look through all your logs to figure out what he's been up to), safety (i.e. undelete but also revert a system as a whole to a known good state).
I remember discussing this with a colleague 7-8 years ago, mostly as a joke (my colleague wanted "everything" to be undoable, not just filesystem operations), and I'm sure others have thought about it too. But what matters is actually doing it...
Looking forward to someone doing the same thing for Linux ;)