Solaris is not open source
This is not exactly news to anyone, but I feel like writing it down anyways: Solaris is not open source, and this matters in practice.
You might well ask, 'but what about OpenSolaris?'
As a sysadmin, the two open source freedoms that I really care about are the freedom to inspect the actual code you are running and the freedom to fix the actual code that you are running. OpenSolaris does not deliver either of those freedoms, as the OpenSolaris codebase is not the same as the Solaris codebase (and I'm not sure if the two were ever the same). In fact, you may well not even be able to replace Solaris things with bits from OpenSolaris without a lot of work.
(That was our experience when we ran into mountd issues; we tried building the OpenSolaris mountd code on some version of Solaris 10, only to find that there were significantly different library APIs. Possibly going backwards in time to earlier OpenSolaris mountd versions would have worked, but then who knows what bug fixes we'd be missing.)
The truth is that while OpenSolaris is interesting and periodically useful, and it's nice to see Sun release the code that way for inspection and potential use outside (Open)Solaris, it's not the same thing as having a real open source Solaris. And Solaris is not open source, not in anywhere near the same way that something like Ubuntu, Red Hat, or FreeBSD is, and this difference does matter to me.
(It may well not matter to developers. Developers likely care much more about the technology and the ideal code than they necessarily care about what exact code customers are running; for most reuse purposes, OpenSolaris is perfectly good. It does mean that there is no way for outside developers to contribute directly to Solaris, but I suspect that Sun considers that a feature.)