It looks like they've added some reinforcement to the joysticks, and changed the connection with the main body to be magnetic instead of sliding in and out (which causes wear and tear on the connectors over time). I hope the Switch 2 is more robust than the original Switch.
Some extra horsepower would also be appreciated. Recently we were trying to play Switch Sports with 4 players, and even my kids who are generally oblivious to graphical fidelity and framerate were complaining that it was basically unplayable in 4-player split screen.