I see. I don't know that much option file systems but I heard some people saying recently that HFS+ is getting old, and they wish Apple would go with something like ZFS. But unfortunately, no progress yet.
What I want to know when the say HFS+ is getting old is what features they think it is lacking.
As for ZFS, what will that really gain you on a small one or 2 disk system? ZFS does have its uses, but most Mac configurations wouldn't really benefit from it in my opinion.
If Apple did adopt another filesystem as the main filesystem for OS X imagine the compatibility issues for moving external drives around. Also, would you have OS X change the filesystem on an OS upgrade or would you wind up with some people using one filesystem and other people using the new filesystem? This would become a support nightmare.