Angry Birds also kept its addictive simplicity as the levels increased, which I don't think the OP mentioned. Plants vs Zombies is another addicting game which shows you as a human desperately planting plant shooters in your backyard to defend against invading zombies. The main difference I can feel is that while both games get more difficult as the levels progress, Angry Birds doesn't make it more overwhelming (you still shoot 1 bird at a time at a more complex building with pigs), but PvZ gets super overwhelming with too many plant types to choose from, too many types of zombies, too many factors to manage to the point where my old iTouch lags because there's too many things going on at once. This loses the user in the mess.