I like my onions like I like my lists:
First - Controls:
- The movement of the ship has to be fun/challenging/intuitive, giving the player a sense of accomplishment when they complete a level.
Second - Well placed collectables and obstacles:
- To further build on that sense of accomplishment, collectables and obstacles need to increasingly test the players control of the ship.
Third - Varied levels & increasing complexity:
- Make the levels more challenging by adding more/new behaviours to obstacles.
Fourth - Quality of life additions:
- UI for score/fuel/time
- New abilities for the ship(shooting/temporary invulnerability)