The ordinary game is pretty damn good, but all the tracks exist in one "world" and your character + kart are racing around it on autopilot in the background of the main menu. You can press a button to dismiss the menu and take over the kart, and start a free-roam mode of the entire game-world with all kinds of challenges and such to select from.

I'm not quite sure it displaces Double Dash as far as straight-up obsoleting it, but it's the first I've seen that brings enough good new stuff to the table that I'd at least sometimes choose it over DD, all else being equal. Every other one I was like "this is OK but I'd sorta rather just be playing Double Dash".