It's in part patents. My understanding is that btrfs does not get around them.
This is not a vague patent threat, because there was an actual lawsuit with patent numbers spelled out, namely NetApp v. Sun. You can check expiration date of those patents. More here: http://en.swpat.org/wiki/NetApp%27s_filesystem_patents
Although I don't understand the following if the issue is copyright or patents:
1. If it's copyright, then presumably a team can black box reversed engineer ZFS, rewriting it under the GPL.
2. If it's patents, then how would any other CoW filesystem (say btrfs) get around them (curious sub point: when would those patents expire)?