Business Week has an article on what makes a great game:
It’s vital to ensure that the player understands exactly straight away what they have to do. You can’t assume they understand ‘game logic’ that walking mushrooms are for jumping on, or that it’s good to collect stars. Obviously a familiar activity such as pool or a big movie brand can help with this, but anything else, from platform jumping to camel-bouncing needs explanation. Spell out exactly what has to be done and what buttons are required to do it, preferably show with a video or tutorial level. Dont imagine the player will read the help section.
This doesnt necessarily mean one-button or even single-thumb but you simply can’t have a control system that gets in the way of instant gratification. Ideally the game should be playable on a four-way joypad without recourse to keys (unless its pressing keys in sequence), but it could equally involve something more creative such as using the camera as motion sensor or speaking into it.