For being a first game I'm really impressed. Gameplay is really good, and you have managed to perform really good graphics and music. Good work. I'm also impressed that the game runs so smoothly, even with many enemies.
Having that said there are two things I want to say before I start pointing out flaws. Firstly, I don't care if it's your first game when I write this. I'm interested in a good game and not excuses if it is not. Secondly, you should use the beta test function on this site before releasing it. I'll judge this as a complete game and not it's potential. This is the main reason why I'm holding back on the grade.
Let's start with the graphics. While the actual game has really good graphics, there are some things that destroys the illusion. The fonts are really ugly. Make some customized fonts, or at least do some serious rework on this part. This goes for all scenes: title, game and end. Another very bad thing is that when you die you are instantly taken to the end screen. No end explosion. Nothing.
One thing that would have looked good is if there were some animations to the explosions and not just a fade out. This is not necessary, but would give a nice boost. However, include an option to disable it for slow computers.
Controls should be customizable. I especially didn't like the shift button. I'm using caps lock as an extra ctrl button, and you know what C-w does in a browser. Yep, closes the tab.
The game is a bit to intense in the beginning. I think you should make the first minute a bit easier.
Powerups would be nice, but don't overdo it. One thing you should consider is an upgrade system, but be careful if you do that. It could easily change the whole game, so make sure that it's what you want if you're gonna do it.
Overall, it's a really good start. If it was a complete game I'd say 4 or 4.5, but since it's so far from being ready I can't give more than 3. However, I've never given such a high grade to a game as unfinished as this.
Keep up the good work. Now make this one a classic.