OpenSolaris has some really nice features, including:
- ZFS, an advanced file system with an integrated storage manager
- DTrace, a universal tracing tool
- Doors, the best IPC facility out there for Unix/Linux
- Service Management Facility
- Solaris Containers
(Wikipedia has articles about all of these. The article about Doors is named "Doors (computing)". Disclosure: I started that article.)
I'm not sure about SMF, but IMO the other items in that list are better than their Linux equivalents (at least for now -- when btrfs is officially released, I expect it to outshine ZFS).
Features like these are aimed at people running high-end servers, not desktops. If you ever have to use Solaris, as opposed to administering it, the first thing to do is install the Gnu userland utilities.