Many people, especially KDE fans who exhibit the fold-and-merge attitude, think that GNOME is a bad KDE clone. Usually, people don't even know what the differences between both of the projects are.
This is written mainly from a GNOME point of view since I'm a member of the GNOME project. If you want anything added here that doesn't justify having its own writeup, /msg me and I'll add stuff to the list.
- GNOME's GUI toolkit, GTK+, is based on C and a simple run-time implementation of objects and classes, unlike Qt which is programmed in C++ with some additional keywords that are interpreted by the Meta Object Compiler, moc.
- GNOME hasn't got an own, official window manager, to the contrary: window manager independence has always been an important goal. KDE, on the other hand, was originally supposed to work with KWM and only KWM. Today, however, both GNOME and KDE are supported by about a half-dozen window managers each, and there is a semi-official GNOME window manager (Sawfish)
- GNOME's panel has always had panel applets. KDE's features applets only since version 2.0, which means that the GNOME applet culture has brought forth many more applets to date.
- GNOME's file manager, Nautilus, contains many clever hacks not found in Konqueror (and vice versa, I suppose?)
- GNOME has had widget themes for years, KDE has started supporting them with version 2.0.
This writeup was renoded after being nuked from its original node; the responsible god offered me to put it here, so I did.