I have been downloading free iOS games to my iPad. Some have been fun. Others were disappointments. I think the deciding factor was how easily I could figure out how to play the game. Take Superbrothers: Sword and Sorcery EP. It has a bunch of blocky style stick figures as characters. There were some instructions on how to interact with the game to begin. However I never really figured out what double tap or hold down your finger meant. Thus I was confused the whole time I played the game.
I had the exact opposite experience with Where's My Water. The initial premise was easy. Swipe to dig through the ground. The levels were set up to gradually teach me more complex techniques. Mostly the hand movements were the same. It was a game of strategy. A new control or two poppoed up every now and then. However the design had me pretty confident at most times that I was in control.
That's what it boils down to. Master designs make it seem easy. I am sure a lot goes on behind the scenes to make sure I learn quick and feel in control. Now I just need to implement that feel in my own games. Then I shall rule.