I think it's kind of odd that you can upgrade your speed (at least it seems so) but one slow-down powerup puts you all the way down.
It is possible to get enough speed powerups and just say "to hell with it" and zip through the entire level.
I would say:
-work on item placement
-work on enemy placement
Make both a bit less random and more pattern-structured. Having random enemies just kind of makes it "ok I guess I'll just blast forward because I have just as much chance of running into an enemy anywhere else." It kind of takes out the suspense when I KNOW that an enemy will be anywhere, so a straight line is as good as any.
Having "enemy formations" in a broad sense, and risk-reward item placements (speed up or fuel powerups in the way of a squad of enemies) makes it a challenge and forces the player to make gameplay choices that add to the experience (do I go the safe route but run the risk of hitting a slow-down? or do I risk death but get speed-up or some fuel?). Choices like these make a game fun.