variable quest rewards for variable performance?

I'm back, fresh from another bombing run through the dead scar. As a daily, I can just about do this one with my eyes shut ... as it is I end up killing off a few extra just for the heck of it. Which got me thinking - how come I don't get extra gold for overkills?

Now, this has got me thinking even further - I can't think of a single quest in WoW which doesn't have a variable reward depending on skill - no wonder players strive to optimise their effort rather than improve their skill. Actually, there are two that come to mind: the Dire Maul tribute run, and the Zul'Aman timed event .. both provide more rewards depending on how you go about them, and a highly skilled (or more likely a highly geared) team will reap more rewards for their efforts.

Seriously though .. can you imagine the deep psychological effect of a wholesale changing of quest rewards to a system depending on skill-based accomplishment?