D-BUS brings us better interoperability with many other programs. While DCOP was pretty much restricted to KDE applications (yes, I know there were C bindings, but not many people used it...), D-BUS already comes with bindings for several other major frameworks: glib, Java, Python, Perl, Mono, etc. D-BUS has been designed from the ground up to be an interoperable IPC system and also to replace DCOP when the time came. And so it did.
D-BUS also allows us to better talk to our own system: projects like HAL and Avahi are already being used by many Linux distributions to let normal applications get access to some privileged resources. In time, I also hope the Portland Project to come around and use D-BUS for its IPC needs, thus freeing us from using a special library with its own protocol to do what D-BUS already does.
You may have noticed a pattern in the links above: all are "freedesktop.org". So, yes, KDE is collaborating with the free desktop initiative. But D-BUS isn't restricted to freedesktop.org projects! In case you didn't know, the Nokia 770s use D-BUS extensively for its own internal IPC. There's also the Tapioca Project using D-BUS. Those are just a few: there are many more (and that list is far from complete).
And, of course, KDE now. (We probably bring more applications into D-BUS in one go than there currently are...)
Many thanks to Simon Hausmann, Harald Fernengel, Kévin Ottens, Benjamin Meyer and Roberto Raggi for helping me with the KDE Libs port to D-BUS. And many thanks to Trolltech for letting me develop and maintain the QtDBus bindings.