Just read this review of Solaris 10. I have not had a chance to get my hands on 10 yet. The review points out some pretty cool new features. The one I like the best is Dtrace. If you have ever used Solaris you know that the truss command can be pretty useful in debugging programs. But it had some limits. Dtrace takes this concept much further. Being able to look inside your program while it is running can save a lot of time when it is misbehaving. Things like why aren't my libraries loading, what's it looking for etc.
The reviews goes on to mention things like self-healing(new Solaris service manager), virtualization(containers), and very quick boot times ( although I never really saw the need for this, my servers rarely have to be booted except for patches)
If you are looking for a good server operating system, Solaris continues to be at the top of the heap. I like Linux ( the blog is hosted on a Linux server at my house) but I don't think it is as far along as Solaris. No doubt though that Linux's capability will continue to rise.