It's really just how it feels (skating vs actually running) and how it looks. In Adventure 2, for example, when you push the control stick forward, Sonic "gets his footing" and slowly gains speed; he takes many steps before reaching that "max running speed" animation. Instead of simply going faster, he moves faster. In Generations and Unleashed, he kinda jogs for a bit, and then when he reaches that max speed, he's taking these long, striding steps. It doesnt look quite right. As for feel, well......try running in a circle in both games, and you'll see what I mean. It just feels like Sonic is actually on the ground in those early games. He has weight to him.