Sorry for the late answer to your original question about why we should care about Illumos, but here's my take and some personal observations / conclusions to boot:

  With a huge sustained effort by many of the people who wrote and/or maintained it, Solaris was finally mostly open-sourced just before Sun died.  There was so much good technology in the OS that it would've been a real shame to relegate it to the Oracle people who would (did) eventually close it, effectively killing Solaris.

 Illumos is the OS/net piece, not a distro, and takes the reins from the moment that Oracle re-closed Solaris as the base and repo-of-record for a number of distributions, such as the ones mentioned above, OpenIndiana, OpenSXCE, Tribblix and most importantly to me, SmartOS from Joyent.  I don't (yet) work for Joyent, so don't take this as a sales spin or anything -- it's just that the company I worked for last year underwent a rather amazing and inspirational cultural as well as technology / performance growth when we switched the stack from CentOS to SmartOS.  It was really cool to see the eyes opening while working with some pretty brilliant programmers who effectively 'grew up on linux' when they saw how engineered, featureful and just generally "sorted out" SmartOS was.  They started using dtrace to dig into performance problems, smf to manage services, speed of containers / zones instead of full linux VMs everywhere, real (not hypervisor-emulated) I/O using zfs straight to SSD arrays, etc. They saw that, in fact, a well written and architected operating system actually does matter quite a lot for their day to day work.

  And it's all seriously free and open source.  Like, Real Actual unix, not a clone.  The way it was meant to be.  They actually really care, not just about being "good enough", but about continuing to develop the operating system in an artful, elegantly simple, efficient and practical way.

  This, to me, is the perfect contrast to the Linux culture that seems to wantonly embrace the 'copy/paste coding' / 'bazaar' / 'good enough' mentality and seems to do no innovation but just produce knockoff, un-architected partial functional clones of what the unix people invented.  But they miss the interacting, distributed, tolerant and very polyclutural computing environment concepts every time.  I think they're effectively dragging the unix ecosystem into the shitter.  Not that this hasn't been done before, but at least before it was ill-thought-out litigious zeal, not junk code and minimum-viable-product quality problems.  This brings to mind the one major, albeit non-technical, problem I recognize Linux (actually, perhaps it was just GNU) as having truly solved - they made it really stinking clear that your license has to be open.  OSI approved open source.  And that accomplishment, of itself, makes it totally worth tolerating its existence.

  Please accept my apologies if I've offended you with the opinionated content there.  These are my honest observations after four decades of doing this, but may seem like flame bait to many.  I don't write about this stuff often and I figure: if anyone's going to be able to understand and respond with constructive input to this train of thought, it's the unix historians here.

  Anyway, a friend I admire quite a lot took the time to put some of this Sun history and Illumos fork stuff into a talk at LISA a few years back and I think he did a really nice job.  Have a look if you like: www.youtube.com/watch?v=-zRN7XLCRhc

  Some other interesting reading from another brilliant guy I worked with a little, who got so disheartened about the failure of tech -- partially Linux monoculture, partially other sad moves -- just walked away from tech to farming instead: http://dtrace.org/blogs/wesolows/2014/12/29/fin/

  Then there's Kamp's recent article: https://queue.acm.org/detail.cfm?id=2349257

Thanks for your time and interest; In earnest,

jake