As rasur mentions, illumos has inherited the Solaris heritage -- and added quite a bit to it besides.[1][2] There are now several vibrant variants of illumos, including SmartOS[3][4], OmniOS[5][6] and DelphixOS[7] -- all of which are open source, and all of which feature advancements in the core differentiators of the system including ZFS, DTrace and zones. Because we collectively have been entirely separate from our Solaris legacy for over three years now (and because we are entirely open source where Solaris is entirely closed source), the differences are actually substantial -- and I don't really think of our system as "Solaris" any more than I think of it as "System V" or "7th Edition" or anything else in our long and complicated ancestry...
While I haven't had a chance to use illumos in any of my projects, the Service Management Facility (SMF)[0] and Fault Manager Daemon (FMD)[1] are two big reasons why I'm looking at switching from FreeBSD to an illumos variant for my home server.
It's been a little while since I've used Debian, but I don't believe any systems comparable to SMF or FMD have been implemented[2].
Not directly (Sun open sourced v28, but encryption shows up in v30), but there are other solutions to that problem. e.g. FreeBSD has geom_eli which can happily underly ZFS or any other filesystem (http://www5.us.freebsd.org/doc/handbook/disks-encrypting.htm...)