Hitbox detection is off, and the physics don't feel nearly as good. There's more to flappy bird than a bird moving through pipes.
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.
:)
I just re-downloaded it from my purchase history, and it seems much better than it was when I originally downloaded it. I had deleted it after maybe 10 minutes because I found the hit detection to be so infuriating.
I'm still bothered by the fact that a game with a pixel art style would have blurry graphics.