A) Simple controls that are easy to grasp yet really takes some time and skill to master (I would personally have preferred something not as "slow" as a spacebar but there's wrapping options out there so I couldn't realistically consider this bad controls by any means)
B) Unobtrusive music that fits the atmosphere and doesn't distract from the game
C) Objectives that gradually increase in difficulty over time instead of a massive brickwall after a few levels
D) Options for levels in case of being stuck on a particular stage and moving on.
This game is a prime example of using K.I.S.S. to make a game that's engaging, fun, simple and yet incredibly difficult at the same time. Extremely well done.