Flappy feels very polished, like if the tap caused him to lift even a pixel higher it wouldn't be as good of a game. This sort of thing doesn't come elegantly, but generally requires a lot of tuning of little variables and ends up farting a bunch of magic numbers all over your programs.

A game like Flappy seems like it should make a good demo app since it's simple, but it actually makes a terrible demo app since the "simple version" is so terrible that you almost feel like you should blame the platform.

:)