One of the things that stands out to me personally is that Illumos/ Solaris values debugging (both in-situ and postmortem) far more than other platforms. Both ZFS and dtrace are brilliant examples of this as well as mdb but I'm much less well versed in the later. If you're interested I'd recommend browsing through posts of perhaps to two largest Illumos deployments.

https://www.joyent.com/blog

http://www.circonus.com/blog

Solaris had a lot of useful and unique features, and was backed with a substantial support apparatus. If you developed a feature for Solaris that got in, it'd be documented and supported to quite a high degree.

If you wanted support from the primary vendor of your OS, and wanted a pretty high-quality Unix, Solaris would fill that niche for you. There's definitely a market for that, but not nearly enough to keep Sun or Solaris alive.

OpenSolaris did not accept contributions.

That's the thing when you look at it from today's standpoint and you are correct there. But jump back to January 2005. You got DTrace and ZFS and branded Zones. Way ahead of the curve. Just look at that release. But sadly it all got overshadowed by Oracle.

And the service/init management stuff, that was long before systemd.

Stability.

a.k.a. stagnation?

No, the ability for the machine to not crash or need to be rebooted for long periods of time. Linux has gone completely downhill in this respect for the past few years. Switching to Solaris on my main Unix workstation has been a breath of fresh air compared to the constant naggering from Linux systems for updates, but I like old school...because it seems to 'just work'.

Many Linux systems now support livepatch to update the kernel for security fixes without rebooting.

ZFS storage layers built on OmniOS/OpenSolaris/Illumos enjoy more years of maturity (and thus demonstrated stability) than upcoming versions for Linux/Ubuntu. If you have to build large storage in-house, stability is a high priority.