Super Mario World is one of my top 5 games of all time. There are days when it's number 1. Sure, it doesn't have the pure, old school platforming that the NES Marios had, but what it has is better. Super Mario World was headed in a direction that Nintendo would embrace whole-heartedly with Super Mario 64, in that most levels in Super Mario World have a playground element to them. The levels all invite you to ask, "What's up there? What's over there? What happens when I do this?" and then they reward you for finding out. Previous Mario games invited you to ask these things too, but not as much as Super Mario World.
In addition, Super Mario World feels better to me than any other game ever. The Mario franchise in general has the best character physics in all of gaming, but Super Mario World is the cream of the crop. The way Mario moves, jumps, and flies is just perfect. There's no other word for it. When I play Super Mario World, the controller disappears and Mario and I are one. And we make beautiful music together.
I probably got too gay there at the end...