For a game of this style, the most important feature is going to be precision of movement and accuracy of maneuverability. As with any video game requiring sharp motor skills, the responsiveness of the controls and translation of those commands to an action on the screen have to be finely tuned.
The second most important will be level design. Early levels should be relatively easy and act as a de facto tutorial for first-time players and a warm-up for more experienced players. Mid-levels should present a mild challenge for experienced players and a learning experience for newer players. Late levels should be a challenge even for experienced players and act as a demonstration of all skills learned during the previous levels.
Third most important feature would be penalty for failing to complete a level. There is a range of ways to handle this such as repeating the level, returning to previous level, returning to level 1, etc. A game doesn’t strictly have to be one or the other, as game options can allow for selection of such things. For example, new players can play a softcore version that allows the repeating of a level until it is mastered, while others can select a hardcore version where you must complete all levels in one life as failing returns you to the beginning, or simply gives a “game over.”
Fourth most important would be power-ups. This is actually very optional depending on the scope of your design. Power-ups that could be included might be temporarily slowing down the ship’s movement speed by a predetermined factor, granting invulnerability for a set number of hits or amount of time, allowing a one-time-use item to destroy a wall on contact, etc.