The game is fun but a few elements need to be modified, firstly you have the invaders traveling across the screen, which in effect makes them very difficult to deal with since every so often you have one or two that speed between the ones your firing at so you are unable to grab that, one possible variation could be that you have them move at a constant rate and increase their life so you need to shoot them several times, secondly, you should add in different upgrades like increasing the power of your gun or maybe from time to time an enemy drops something that changes your attack completely. Heck you could even have it so that you have to get a certain type of upgrade dropped by an enemy to upgrade that set of upgrades. A third thing maybe is that you could maybe put a little more work into the look of the shot lol, i mean everything looks so great and then when it comes down to you shooting it's just a triangle that changes color lol. Otherwise i think you've got a good game going here, it looks good, and it plays well. a little work and it could easily be thrown to front page. Goodluck.
ps. oh and if i wasn't very clear with any of my suggestions feel free to pm me,