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You couldn't go all the way to HIG-ifying KDE, but you could get pretty far. You could do themes and icons, of course, but beyond that you could redo all the toolbars to be simpler, redo the menus to be more logical, and redo all the context menus to be more streamlined, all without touching a line of code. Beyond that, you could ship simpler default views for Konqueror (eg: no sidebar), a KControl with fewer options (each pane in KControl is an independent component), etc. At some point, you'd have to start touching code (eg: to redo widget layouts to be more friendly), but the KDE folks are working on improving those things already. I suspect you could at least make KDE vastly simpler (if not as simple as possible), through non-code related changes.