I liked the simple graphics style, and the sounds...
Others have mentioned some flaws, so I'll skip those except to say that in and of itself, not being able to move while shooting isn't necessarily a bad thing. It was the needing the "standard" jump key that made that bad, the aim being "enter" or using mouse-click-down to aim (stop move) and mouse-click-up to fire (move again), would be better.
Since you can fall off "inside the area" and live, I expected the same for the "outside" (even though that wouldn't be the normal expectation) and well, I was right, it doesn't kill you, instead, you just fall forever. Dying would have been nice. ;-D