I like how although simple, game graphics convey what they have too with a sufficient amount of juice to make it entertaining.
On the difficulty / fun side, it is was bit too simple. By going in either left or right and doing a sine wave movement (slowly up and down), I won the game by seeing almost no enemy planes ; the spread caused by the sine movement was way too efficient.
Maybe enemy types, to create some chaos and force you to change your strategy (à la Doom or even Luftrausers).