I am (was) a Solaris admin through to about nine years ago, from v2.5 through v8. I've had a little exposure to Solaris 9, and almost none to 10.
My reasons for avoiding Solaris:
Hardware support is not nearly as good as many Linux or Windows operating systems. It is improving.
You can run Solaris for free, but you can't get updates for free. Not even security updates. Not even 0-day exploits. You have to buy a support plan, per system, which can be expensive. This means that the way to get updates is to wait for the next "U" release, and upgrade at that point.
OpenSolaris is too bleeding edge for me. It changes too often, and the releases wander too close to unstable or unreliable for my needs.
Between Solaris and OpenSolaris, Sun has managed to totally miss the happy medium between "welcome to 2004" and "I'm so new and shiny I don't really work 100%!"
I used to be willing to use Solaris more before the Blastwave project imploded. Through there I could get newer tools that fit more with the Linux way of doing things (which is where I spend 95% of my professional time) with a relatively easy online way of managing the tools and updates. Once Blastwave and CSW settle down, I'll look at both remnants and decide if it is worth putting time and effort into either of them again. Really, the loss of Blastwave as a trustworthy source of tools was a huge blow to Solaris' viability in my circles.
But the number one reason for me is that right now I don't need to do anything that requires Solaris.