I think you're off to a good start. It ran just fine in 64-bit Linux Mint Cinnamon 13, but the very bottom of the window got cut off. This is a problem I often have when games run in windowed mode, maybe because the size of the title bar isn't factored in. My display resolution is 1366 x 768.
I completed the first three levels and almost to the end of the fourth.
ControlsThe controls felt a bit 'slippery' for lack of a better word, as if there's some lag between input and ship motion. The first time I played, the ship would fly straight horizontally when I tried to move diagonally to the top left, but it worked fine when I tried to move diagonally to the top right of the screen. Diagonal movement wasn't a problem the second time I ran the game.
GraphicsI think the vector style gets the job done, but I think the line colors could be more intense. Right now, everything feels kind of drab instead of having a vibrant neon glow. It felt a bit too blurry in motion and I think that contributed the most to muting the colors. Most importantly, it made the fast-moving bullets hard to see.
My favorite enemy pattern was the ones that alternated between shooting lasers and v-shaped bullets. They had an interesting movement pattern, the colors were interesting and more varied. The laser beams were nice and vibrant, so if you applied the same effect to the rest of the graphics, I think it would help a lot.
I like the style of the explosions, but I felt they were a little too intense. I think it would be nice if they faded out more quickly, especially since they're the most intense of the elements on screen while at the same time being the least important element. I think it would make more sense if bullets, which are most important to spot, were more intense and visible.
DifficultyThe speed of the enemy ship patterns varied a lot, something which made the difficulty curve seem very uneven. There would be really slow ships and then suddenly really fast ones. If they gradually got faster, it would have been easier to adjust. I think there could still be faster enemies, but when most of the other ships were really slow, it seemed jarring.
The bullets were sometimes very hard to see and avoid. It felt like there was no way to avoid getting hit sometimes and you just had to deal with taking damage, something which felt a little frustrating since it didn't feel like something which would be possible to avoid with a bit more practice. This was especially a problem with the pea-sized green ships. The larger enemy ships with larger and slower bullets were OK, so I think larger waves of those would work better.
Event feedbackThe bullets I fired felt weak and ineffective. When I shot at smaller ships, I couldn't tell if a) the enemies were really hard to hit and took only one shot to kill or b) if they took more damage and death was the only visible indication. I think it was mainly that I was firing so many small, fast bullets that they were just a blur and I couldn't really tell if they did anything. It was easier to tell that it was case b when I shot bigger ships, so I definitely think hit indicators like tiny explosions are needed. I also think fewer, bigger and slower, but more powerful bullets would have felt more effective if I could have seen the impact more easily.
It also seemed a bit strange that my ship bounced off the enemy ships after collisions. A lot of the time, I hadn't realized I was colliding with anything and suddenly my ship was moving in the opposite direction. It took a bit of getting used to. I can see the use in having the ship move away from a danger zone like that, but I think some kind of color change or something similar would have been less jarring.
PacingI think the waves of enemies could have been more closely timed in some of the levels. There were pauses between waves that just felt empty instead of being a needed break.
OverallI think it has potential, especially if you can iron out the wrinkles and come up with some interesting mechanics to set it apart from similar games. Looking forward to seeing this progress.
Developer
