As Wolfgang Lonien recently pointed out, Solaris isn't the only system with ZFS, as you can also get it in at least FreeBSD. Despite these ports, I believe that Solaris generally remains your only solid choice if you need ZFS.

The problem is that ZFS is currently incomplete, with both missing features and significant problems. This means that the work of integrating ZFS into other systems is not complete, and there are two potential issues with the remaining work.

The first is the most straightforward: you are taking a risk that Sun will change the license terms of future, more complete versions of ZFS in a way that leaves the code not usable by your operating system. This would maroon you with the current incomplete ZFS.

(I don't believe that this is a big risk, but I am not convinced that Sun really gets open source in its heart, and I am pretty convinced that they see ZFS as a core weapon in their commercial strategy.)

The second is that what operating systems will need to put in work in the future to track and merge Sun's upstream changes. This is at least time consuming, delaying how fast fixes and features reach you, and you run the risk that it simply won't get done, that the necessary developers will run out of interest.

(My impression is that integrating upstream changes is a tedious, unglamorous job at the best of times. And I'm not sure that any OS will really commit itself to ZFS given the various uncertainties around it, which lowers maintainer motivation.)

Or in short: the current state of ZFS is such that other operating systems can't really just fork the codebase, ignore Sun, and go their own way. Keeping up with Sun will take effort and time and means that Sun could in theory cut people off, so what you see now in other OSes may be all you'll really get.