There have been, however, important developments that come from the increase in processor and graphics power. I mean, wasn't Mario 64 the immersive, exciting next gen game it was due to the sudden addition of a third dimension? I suppose advances like that have been few and far between lately, but doesn't the addition of online play increase the scope of a game? The internet on a console still seems next gen to me, since I don't have an Xbox.
Taking your racing examples: if you gave the original Mario Kart or Gran Turismo the modern graphics, they would still not be next gen, since it wouldn't surpass our current expectations of what a game should have. The next gen is something that hasn't been necessarily been seen and done before, whether that be gawking at the greater detail of the graphics, wondering at new advances in gameplay, enjoying new and accessible control schemes, or just being taken in by a gimmic. It's all connected to something shiny and cutting edge--it's why they sell so many of the new ones, and why they aren't still selling Ataris and SNES.