The controls have some oddities. There seems to be a slight acceleration/deceleration going on which screws things up. I'd say it's generally a good idea to leave that kind of things out unless the gameplay is based on momentum like sonic or mario. It's pretty hard to get to work properly. Now it only adds confusion. In addition it feels like it gets worse with diagonal movement. Small nudges to dodge things are hard cause the ship might move just a few pixels or then it might jump a longer distance. Slowing down the ship might help a bit, but it wouldn't remove the fundamental weirdness. In life force on NES it's easy to navigate through small openings even if the speed is maxed. There are no "extras" in the movement. Pressing a button makes the ship move on a set speed, that's it. Moving in a circle, going back and forth left/right up/down really fast should make the problem apparent in your game.
I'd say the controls are tight when you can clear out a scattered wave with a single bullet for each enemy with no misses
One thing that helps making the controls more responsive is giving priority for the latest pressed key. Since it's played on a keyboard there will randomly be a situation where two opposing directional keys are pressed down at the same time. Now the left key always takes priority which might give a small latency when switching from moving left to moving right.
The art looks nice, but one thing that bothers me is how the front of the ship flickers like mad when moving horizontally. There's a space where adjacent pixels alternate from dark to light. Making the other spikey thing darker probably fixes it.
You could also experiment with different types of explosions and sound effects to work on the really top-of-the-pyramid game-feel territory. The way some specific explosions were made in silkworm are pretty neat and i haven't seen it in other games. When you shoot the mines or bombs or whatever they were, the explosion is really fast and moves away from the direction it was shot from. It really feels like blasting something away. Generally a lot of shmup enemies' movements and explosions don't seem to be affected by the momentum of your bullets. Maybe giving it a feeling of blasting away stuff could add some immersion? As for the sound department, smashing the random button in SFXR should give you enough resources that you can mix together to create an explosion sound that really feels like something specific, even if you can't tell what it exactly is. Try to give it sound that makes killing the enemies feel really good.
The best feeling related effect at the moment is when the bomb is used and a lot of explosion sounds go off at the same time and the screen flashes. Has lots of "OOMPH"
One thing i liked a lot was how the pickups moved up instead of down. Gives you a reason not to shoot all the time. This kind of small things that add gameplay without adding control complexity are more than welcome in any game. Now NOT-SHOOTING is a viable gameplay strategy. Waiting and shooting enemies when they're closer to the bottom of the screen gives time to collect the flickeryballs. Maybe you could expand on this idea with boss battles and such?
I'm looking forward to play more of this game. It's really great for something that was created in such a short time
