LVM and ZFS are barely comparable. LVM provides volume level snapshots, which means, among other things, that you must pre-allocate your snapshot space ahead of time. Basically, you must anticipate and carve out how much space you think you'll need to hold your snapshot data. This makes it significantly less useful in my experience. I'd compare ZFS snapshots to FreeBSD's UFS snapshots or Microsoft's VSS. But, it's better than those. The performance penalty for using ZFS snapshots is nonexistent, for one. The snapshots are instant, and hitless (no filesystem freezes.) There's seemingly no limit to the number of them you can have. They can also be read-only or read-write (I forget if LVM does this or not).
Snapshots aren't all that ZFS does that's unique, of course. There's end-to-end checksums, optional compression, built-in volume management, quotas, reservations, transactions vs blocks, multiple caching options. Mostly it all comes down to flexibility. You can do almost anything you'd want to do with it fairly easily. It makes you look at filesystems from a different angle. It has its share of problems, but the big ticket ones have been mostly worked out at this point.
I'd also like to add that I think DTrace can be a huge time saver in the right situations. I go back and forth between which (ZFS or DTrace) I think is more useful overall.