Something else that only gets annoying after repeated plays: you shouldn't have to click play twice. Just make the quality selection accessible via some menu option, but don't force people to see that screen every time they play again.
Very nice. The only thing that could make it more annoying would be to make the object of one of the two sides to *catch* triangles, not avoid them. How about adding a hard mode that does that?
Please either change the keys to move the tanks, or make it possible for players to redefine them. Not all of us are using a QWERTY keyboard; German keyboards have the Z and Y switched around, for example, which makes the tank thing extremely annoying.