NES controllers have 4 buttons. SMS controllers have 2 buttons each, but some games, like Alex Kidd & Wonder Boy required you to also use the pause button on the console itself.
I agree you don't need a slew of buttons for a good game, but a lot of that design space has been explored already. I've seen some clips of the games in action, and it doesn't look like anyone is really using the crank or accelerometer in fun enough ways (after all, accelerometers have been on AAA console controllers for a while now and are relatively underutilized). I know this is subjective though :)