Simply, the most feature rich, the most friendly and powerful desktop environment in the world. It offers dozens of excellent applications like Amarok - a wonderful music player, far better than itunes, digikam - excellent photo manager and editor. It's a fast desktop environment built with advanced Qt toolkit.
2008 was a difficult year for KDE. Technology-wise KDE 3.x became a dead end. OTOH the KDE 4.0 workspace was not ready for the general public, even though several KDE 4.0 applications were already better than their 3.x counterparts.
The 4.1 and the upcoming 4.2 (currently in beta) releases show clearly how amazing the base technology is. The leaps that the front-end apps make within the relatively short development time are astounding.
Currently the biggest hurdle in KDE's way is NVidia. The latest release version of their graphics drivers contain various bugs that result in very slow KDE 4 execution. The 180 series of drivers is still in beta (180.11.02 works fine for me, btw).
If the few stability issues I currently have with 4.2 beta get ironed out for the final release, I don't see a reason why anyone would want to use another desktop environment.
KDE is an excellent desktop environment. Actually I think it is currently the best. It's easy to use and has a lot of options that enable you to customize it exactly a you want. The community of users and developers around KDE is also great. I liked it so much that I even started learning programming in C++ and Qt so that I can one day help with development.