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