If you want to make a commercial shmup with the goal of making money, sadly I've heard from experienced devs that the PC is a miserable platform for it. Not only do they generally require a game controller to be played properly, which PCs don't come standard with, as I hear they don't sell well either.
However, if you're still determined, then here's the review.
Ditch the acceleration deceleration of the ship. Shmups are the twitchiest of twitch games, and if I press up, I expect the ship to go up instantly. It's not realistic, but it plays much, much better and in these games gameplay is king.
All bullets, bombs and lasers need to be easy to spot at all times. Use high contrast to make them easy to spot (watch videos of Ikaruga to see what I mean). Also, the particles from the explosions hide stuff like the lasers too much. Render threats on top of everything else like explosions.
As I mentioned about twitch gameplay, the controls need to be razor sharp. The same frame the button is pressed the bomb should explode, saving the player from what was about to hit them instead of moving to the center of the screen first.
Serious overuse of particles.
Add some way to adjust movement speed while playing. Einhander was awesome that it could be adjusted without powerups.
Periods of way too much waiting while the boss simply sits there.
Needs more impressive weapons (I assume you simply haven't made them yet).
Thanks for your review!
That's really bad news that they don't sell very well :< I was actually considering making it for an iphone, but my circumstances pushed me for solution that is quick to realize. Let me ask you a question than: what would be the best selling 2d games on pc atm?
I'll take your advices regards to controls, bombs and lasers. However i do like that amount of of explosions. I might reduce amount of shoot particles thought, and I'm planning on using textures.
I'm planning on implementing shop system where you're buying better gear, so by buying better engine your speed will increase.