This overview of the Linux+ZFS licensing crap is pretty good. It comes from GPL supporters, but it clearly shows what’s so terrible about the GPL. It’s not just a copyleft license, it’s a strong copyleft license:

According to these provisions of GPLv2, if you create a binary work that incorporates components from the GPLv2’d program, you must provide the complete corresponding source for that entire work with the binary, and the terms of the GPLv2 apply to that source

binary distribution of ZFS plus Linux — even with ZFS in a .ko file — constitutes distribution of a combined work, which we name Linux+ZFS

This is completely absurd. A license that infects tarballs?