The harm that comes from ZFS not being GPL-compatible
One of the big issues with ZFS for Linux people is that it is licensed under the CDDL, not the GPL (v2 or otherwise). In the past, I may have sort of said that this wasn't a too big deal because, for instance, there would have been major practical obstacles to getting ZFS code into the kernel anyways. I take that back unreservedly; I was wrong. I have come to see ZFS's license as a real problem for the simple reason that ZFS's license keeps it from being bundled with most Linux distributions.
Now, what makes this happen (or not happen) is not the legal technicalities. Some people feel the CDDL is not actually incompatible with the kernel GPLv2 license, and Canonical certainly thinks there is a legal way around things (since they're doing it in Ubuntu 16.04). And as a practical matter, a Linux distribution would probably not get sued. But software licenses are social things too, and the social side is quite clear: quite a lot of people feel that the kernel's license and ZFS's license clash in a way that means you cannot and should not combine them. A move to do so anyways that's justified on legal technicalities is going to make a bunch of people angry (and has, with Ubuntu 16.04).
I have come to feel that this lack of bundled ZFS on Linux matters quite a bit because ZFS on Linux remains your only real choice for an advanced, checksum-based filesystem. Btrfs remains not a viable alternative and when people are finding serious problems in btrfs's RAID 5/6 code, it's going to stay that way for a while. If ZFS was licensed in a way that was (socially) compatible with the kernel, I'm pretty convinced that a bunch of distributions would have already shipped ZFS on Linux in some form and a bunch of people would be using it and thus would have better-protected data.
(I don't think distributions would have integrated ZFS on Linux into their kernel source. But they could have shipped it as a separate package with precompiled kernel modules and so on, and the odds are higher that they would have supported it in the installer as an option for user filesystems.)