Stefan: Awesome to hear you were pushing GNOME at Linuxtag. For what it is worth, the answer I usually give when asked the difference between GNOME and KDE (for users) is simplicity- compare, for example, nautilus’s right-click ‘burn to CD’ for an iso with the incredibly powerful but difficult to use k3b, some simple preferences menus (like nautilus’s v. konqueror’s), or the necessity for search functionality in the KDE control center, or GNOME’s leadership in developing desktop software that Just Works, like hal and network manager or tomboy and muine. When giving this answer, it is always important to stress that this isn’t for everyone- I think this makes GNOME clearly better for me, and for my mom (and hence in the long term for hundreds of millions of people), but we should be honest and up front that this approach is not necessarily for the average hacker or admin who likes to tweak every little setting.
For developers, of course, the answer is LGPL and language support- python, C++, and C# are first-class languages for us, not just C, and Java is catching up, which means you can do work in just about any language you might want. Again, it is worthwhile to admit that right now have better tools for development, but hopefully we’re catching up in that area as well.
For businesses and big deployments, the answer is ease of use/simplicity, which means cheaper support and lower training costs, and corporate support- Sun, Novell, Red Hat, Nokia, and perhaps now Palm are all helping develop our software, which improves the experience for everyone, and providing support for customers who want that level of expertise.