OS X's Time Machine product is pretty straightforward -- it does do a weird trick with
hard-linking directories, but one could write a perfectly great desktop backup program without
any such hacks. It's not really complicated and btrfs doesn't really make it any easier. The
problem is that for some reason, people seem to only be interested in writing backup systems
that are either really grungy and low-level and often inadequate in practice, or all-singing
all-dancing incredibly complex things. I guess this is because the two markets for backup so
far are "geeks who want to hack something together for personal use" and "full-time sysadmins
who get paid big-bucks to manage large networks".