We tend to hope that we will find the perfect game; that there is some formula for creating the best, most addictive game possible. And whenever we have a new hit, those hopes get projected onto it. In recent times, the perfect game has been thought to be games like World of Warcraft, FarmVille, Candy Crush. And then another game comes along. Charles Pratt and Tadhg Kelly have made similar points. But I think it goes further: people play Flappy Bird because it flies in the face of what every game designer knows at this point. Not because players care the least about what game designers or theorists like myself think, but because the shared conventional wisdom of How You Shall Design Your Game is making games similar, and players know a breath of fresh air when they see it.
Juul on Flappy Bird is comprehensive, interesting and to-the-point.