In terms of the basic desktop layout, KDE more-closely follows a Microsoft Windows style with a taskbar, start menu, and various other things. On the other hand, GNOME takes the Mac OS X approach, placing the menu bar on top of the screen and a separate bar for currently-open applications (the Task Bar).
KDE and GNOME each have their own set of usability and accessibility guidelines:
While GTK is a very popular toolkit for Linux and thus most apps have better integration with GNOME, KDE's interface is normally considered slicker and more polished, possibly due to the underlying Qt toolkit.
