I'd like to see Apple revive some long-overdue technologies from OS/2's GUI (like drag and drop fonts and colors - something I've yet to find in any other OS but which made working in OS/2 fantasically easy and powerful all at the same time). Windows "kind of" has it with their Display > Properties > Appearance settings, but you cannot, for instance, set individual apps/windows to use different fonts the way you could in OS/2. Heck, I'd be happy just being able to change the default fonts (for menus and toolbars, dialog/sheet text, etc) in OS X. The zooming capability is handy, but I don't necessarily want to pan-and-scan my desktop just to have fonts show up bigger.
Useful improvements to the system such as these would certainly make it worthwhile to pay full price for the upgrade. New versions of existing web browsers and chat clients are not (for me) sufficient reason to pay full price for an upgrade.