Interoperability with Windows is even better now. Samba seems to work really well. There is Exchange support, and VPN access to Windows networks is there too. On the third disk you will also find a package with support for Common Access Cards.
Interoperability with traditional Unix is also upgraded with the inclusion of XFree 4.3. I was able to run Gnome and KDE in a Mac OS X window (via Xnest -- slower than the rootless X11, because Xnest doesn't support fontconfig) or directly on the rootless X11/MacOSX shared desktop, or on a "virtual" desktop running in full screen mode on top of OSX. Good stuff, though too bad that 'xclock' won't load though for some reason.
The preference panel has been re-worked a bit, the OS now has much better Bluetooth support, more printer and scanner drivers, USB 2 support, support to setup your Mac as a Remote Desktop Client, telnet/ftp/personal server abilities, print and internet connection sharing and much more, easy to setup and use via the preference panels. TextEdit can now read .doc files.
The UI has also seen a refinement. Widgets and windows are all clean-looking. I like the new tabs (they look like buttons now), I like the new tab views, the lesser trasparency in the menus, the loss of these (ugly on LCDs) horizontal lines on window backgrounds etc. If you are using a metal application and you invoke an alert or an "attached" child window on the master window, the effect of the way the child window pops up out is impressive (see it on Path Finder 3.x as well when you tell it to customize its toolbar). Overall, Apple did a wonderful job on the UI again. However, Java applications have not been updated to use the new look. They are in fact still using the same look as in MacOSX 10.1 (Jaguar 10.2 also had a UI refinement), and needless to say, they look out of place, even if Apple has the best-looking and most integrated Java apps on the planet.
Developers will feel at home with the new developer tools Apple is offering, XCode. By using clever tricks like distributed builds, zero linking, 'fix and continue' (an SGI innovation some years ago), code completion, fast search, predictive compilation etc. Apple is now able to offer truly competitive tools that should bring new development houses in the Mac platform and enrich it with more applications.
Other new features on OSX include a more automatic iDisk for .Mac users, better text-to-speech quality (still sub-par, though, compared to some specialized solutions I saw a few months ago), better font rendering, updated Address Book with new features, updated iSync and iCal and many more changes under the hood. Oh, I should not forget that the mouse acceleration and speed has been worked out and now we get much faster motion by the input devices. Personally, I would upgrade for just that feature alone as it has given me grief in the past.
Last but not least is Panther's speed. Users with older Mac computers (like my G4 Cube 450 Mhz) will welcome the overall new speed levels and UI responsiveness. File operations seem faster now (e.g. unarchiving, moving files from disk images to your drive etc), launch times are much better, scrolling is better too. While UI responsiveness is still not as good as in Windows XP or other OSes because of the technologies used for Cocoa/Carbon apps, the OS is fully usable and it won't be a problem for most users. Applications that don't use Cocoa or Carbon (e.g. X11 apps) resize and scroll extremely fast, which means that the OS' speed is not the limitation, but the advanced techniques used on Cocoa/Carbon apps (e.g. proper non-flicker algorithms, PDF engine etc). And that's a good thing overall. It is a give and take kind of thing: you sacrifice some UI responsiveness and you get a headache-free and good looking modern desktop.