The ZFS thing still bums me out to this day, but I think I got over the demise of Solaris when OpenSolaris fizzled out.
My vague memories of it are that the userland showed signs of legacy (/usr/ucb), and I could never get the tools I needed installed. But that was a consequence of the setting I was working in, not Solaris.
It would be particularly interesting to hear from fans of STREAMS or other kernel features.
I personally don't give a shit about Java or MySQL, so I wouldn't have been sad if those products died, but I know lots of people who would be.
For me, it was ZFS that I was really excited about. Back then, I thought I might have ZFS (or some ZFS-derived or -inspired equivalent bitrot-resistant checksumming filesystem) on all my computers and phones by, say, 2017.
But I don't. My Linux box can finally have ZFS (with some caveats), but my Macs and phones can't and won't; it was another sad day (for me) when we learned Apple's "modern" new filesystem, APFS, does not do checksumming for user data[1][2].