chimpdudex left a good review. He said you've got a good setup, it just needs to be expanded. This game may or may not survive because it's just soooo... plain.
Graphics : Weak, at best. Clean stuff up... don't worry so much about VOLUME of monsters as making them look good. Details, details... Don't just make the same 3 or 4 creatures and duplicate them dozens of times. Make 10 monsters, use your zoom and add some nice detail, and voila - you're in business.
Your environments were okay, colors and textures were kind of plain. Lines and shapes were all just "first draft" looking. I guess all-in-all you just need details.
chimpdude was VERY right about your attacks. The "black mass" is just sad. Lightning bolts, fire, a tornado, magic green dust, anything but scribbles :-).
Style : Weak, again. Sorry to be critical, you did a really good job for what I'd consider a STARTING game. I really hope you work on it because you have skills. But stylishly, this was just boring. The same two attacks, some simple ActionScript, and a shit-ton of monsters. Side-scrolling got old for me a long time ago, and you haven't done anything yet to make this one stand out.
More than one or two attacks makes for an even better game, obviously. At some point it gets complicated and I'm sure having a 1-5 weapon selection isn't easy. But style would be improved by unique weapons (just look at Madness Interactive and Thing Thing).
Style would also be improved by some kind of storyline. "In a future where the world has finally come together, just to be attacked by aliens," or whatever filler-type crap you want to put in there. Give us a REASON to care about the character (I'm kidding, but you get my point).
Sound : Yikes... at least you HAD sound :-). Sound effects, a good soundtrack, character dialog ("Oh my god, they're coming!!!"), lots of things I'm sure you know that you could add to develop this game's audio.
Violence : Come on, man, BLOOD!!! I don't want my aliens to vaporize, I want them to explode and rip apart from the inside out... lol. And when the character takes damage, same thing. Add some blood and guts, we all love blood and guts. But really, the devil's in the details - and B&G is just in the details.
Interactivity : More features would be ideal. Difficulty selection, tutorial, level selection, save feature, weapon selection, upgrades (all that REALLY difficult scripting and animation shit). I know how hard development is JUST on a game like this one, making it more complex is like a full time job, so know that these are just ideas that you can take or leave. Like I said, side scrolling wonderment can be found in Madness and Thing Thing if you need creative ideas.
Humor : Nope.
Overall : KEEP IT UP. It's a good start but to survive, and definitely to be enjoyable or even tolerable, needs detail and conceptual improvements. I would say the big 3 are - overall detail, add some more simple features, and general audio improvements. Keep working, you have good skills - progress is key to improvement.
My Grade : D+ (I gave you a 3/5 hoping it would live, we'll see).