Traditional shoot'em Ups, shmups, are awkward on the iPhone, because your left thumb covers a quarter of the screen and your right thumb covers another quarter—bad when you need to see every single bullet heading for your ship.
So instead of having to drive your ship with your hand, you drive it by tilting your phone in the direction you want. It's less precise, but you do get to see the entire screen, which is vital for shmups. (You also have the option of using screen press mode, which you'll actually end up doing anyway, for more precision in flying.)
Also gone is the outdated strategy of having to repeatedly press, or hold, a button to shoot. Worldy Wings just has you shooting the entire time, something most players do anyway. You just tap the screen when you want to set off a lightning attack.
With only two difficulty levels, three planes and 5 missions, the game is way too short. But it's $2, and 1942/Raiden fans will love it. [iTunes]