@ariadne @whitequark @mcc I found it mostly just incredibly stable, predictable and well-documented. The man pages were top notch, and the system felt properly engineered. But it has definitely aged a lot and most Solaris distributions nowadays import lots of Linux and FreeBSD tech.
Sun not open-sourcing everything was partially legal reasons with 3rd party contractors holding the copyright, e.g. with i18n code. And then Oracle came and put a nail in the coffin.
But yes, most of the ideas of Solaris live on, be them SMF (upstart, launched, systemd) or ZFS (Btrfs). Like most of Sun tech, Star Office being LibreOffice now, Sun Java, etc.