It depends a bit on what you do and use. If you primarily use a terminal, Firefox and thunderbird there isn't much of a difference between those. Yeah, taskbar etc look a bit different but not much.

If you switch between KDE and Gnome apps (kedit vs gnome-edit, kmail vs gnome evolution, KDE dolphin vs. Gnome nautilus, kmines vs. Gnome mines game ...) you see more of the differences in App Design principles.