Vista->7 was certainly a smaller change than XP->Vista but more than service pack level. For instance, one big one is that DWM in Vista keeps a copy of each win

Vista->7 was certainly a smaller change than XP->Vista but more than service pack level. For instance, one big one is that DWM in Vista keeps a copy of each window (at least old GDI windows, maybe not DirectX ones) in both CPU and GPU memory, which both consumes unnecessary space and generally slows rendering down. In Windows 7, they're only on the GPU. It's a classic example of something changing significantly under the hood but not being very visually obvious.