I'd say it's not the compositing/effects, it's the spacing and general layout. KDE apps tend to look quite cluttered and claustrophobic, at least when I compare

I'd say it's not the compositing/effects, it's the spacing and general layout. KDE apps tend to look quite cluttered and claustrophobic, at least when I compare them to e.g. OS X and some better GNOME apps. A bit more whitespace wouldn't hurt, you can't fix that with gradients and transparency.