Yes, the default KDE themes are like Windows XP "Clown on Acid"(tm), but you can easily fix that, there are about a dozen widget sets to choose from in KDE 3 - my KDE widgets are basically CDE clones - simple Adode Sans MT font for menus and buttons etc. Very flat, minimal 3D, minimal shadowing. The antialiasing in Qt is so good that I can't live without it, and in fact I suppose my primary requirement from a machine of any type is the ability to make print-quality letters onscreen. I have one GTK2 app that has nice antialiased widgets and that looks OK as well.
I think the color-explosion thing is a European taste - could be the drab weather and long winters - I seem to remember continental Europe as being generally garish.