Played it for a bit.. and got bored quite honestly. That would be the games biggest detriment at the moment. Having played a fair few Gradius style (and Gradius games) in my time, I can give you a few pointers:
Under normal circumstances, I wouldn't complain about the graphics, but they aren't exactly... well.. good.
* The overall colour scheme for the game is quite garish to look at. Your backgrounds might want to consider dulling your palette a bit.
* On the other hand a lot of the sprites blend together:
There is a distinct lack of contrast in this sprite, making details very hard to tell apart.
* The quality of the pixel art seems to vary wildly. Some enemies look fine.. others I can't tell what I'm supposed to be looking at.
The graphics are an aspect that can help a game get noticed.. how-ever that is not where the major fault in the game lies:
* Stage design is non-existent, or at least it was by the time I reached where I did. I think this is one of the biggest problems with the game as it stands - everything is just flat. There is little to no variety in the stages - terrain would've been helpful. Take another look at Gradius games and you'll find that, past the first game, they start mixing it up with gimmicks and the like. Closing walls, fire storms, etc. Gimmicks are possible even with the stages you've used, and even then if you don't implement gimmicks, having actual stage design can make up for it. The good thing about these types of games, is that you don't really have to bother with working out collision, after all, everything
kills you. This include the walls, ceiling, etc! Actually add some stage design to your game and not just wave after wave of enemies, and it might actually be less dull.
* Weapon balance seems... off. I picked up Fire Claw once, and outside of dying there doesn't seem to be a reason to use any other weapon - it has great screen coverage, high damage output, and extremely high rate of fire. All this is fine.. but on the other hand, some of the other weapon combos are so weak it takes an age to kill even basic enemies.
* The soundtrack choices are fine for the most part, although some pieces stick out like a sore thumb. When most of the game is rock, and then you have peppy Gradius tunes playing, they really stand out.
I'm hoping I haven't been too harsh, but I figured I'd give you the honest truth about the game. It still needs work... especially in the stage design department.