I'm sure others may answer with specific differences (e.g. our amazing ZFS file system or our DTrace technology or the fact that the arguments to various commands are different or that drivers for hardware may or may not be present); but I have another lens through which I look at this.
OpenSolaris - and Solaris in general - is another accessible UNIX operating system. One used in places where GNU/Linux isn't used - where there are potential jobs that people with disabilities will now be able to occupy. For example, one thing OpenSolaris is particularly good at is scaling to lots and lots of processors running the same single operating system image. EBay makes heavy use of Solaris because the kinds of database tasks they have are best handled by large machines with hundreds of (SPARC) microprocessors all running inside a single machine - something the Linux kernel isn't yet capable of. Solaris is used for similar applications in the financial sector handling stock trades.
On top of OpenSolaris and on top of Ubuntu Linux and on top of Fedora Linux is essentially the same GNOME desktop, with essentially the same accessibility infrastructure and of course the same Orca screen reader (and GNOME On Screen Keyboard and MouseTweeks and ...). As has been noted here, there are differences in the audio subsystem of OpenSolaris and among the various Linux distros (which may be important to users). There are differences in what hardware is supported (which is of course very important if it is the hardware you own!). But from an accessibility point of view they are pretty much the same.
So what I think it means for users is that the same Orca screen reader you have been using - on the same GNOME desktop you have become familiar with - is now available more places, including more places where you might find employment or which you might encounter in a school or a library or...
Oh, and if you have a particular interest in some of the specific technologies available on OpenSolaris - like the ZFS file system or the DTrace dynamic tracing framework for monitoring & debugging & troubleshooting systems - you now have an accessible graphical operating system containing those technologies.
Accessibility Architect & Principal Engineer,
Sun Microsystems, Inc.