This has the potential to be a truly great game ...
Some things I recommend working on:
First off ... work on your movement controls. It was mentioned that this is a somewhat overdone style of game ... the thing your lacking is the front/back movement instead of just the up/down movement. It add a bit more strategy into the game.
I think the idea of adding an upgrade system of some sort is fantastic. New weapons, speed/weapon speed upgrades, engine upgrades, force field, etc ... There's a whole range of things you can do with the addition of an upgrade system.
You animation wasn't bad at all ... but perhaps add a bit more depth to some of your ships? More logo's or a better alien inside the cockpit ... that's more nit picky than anything, but it could improve the overall game quality.
I'd personally suggest putting together a storyline of somesort. WHY are these aliens invading? Is this some kind of a Earth Hero defending against them? Does he have friends? Is he doing this out of a secret garage/hideout? YOu can do so much with a game by adding in a storyline/plot. Break up the game into missions ... give the player a GOAL in the game instead of endless/mindless alien killing until you eventually take so many hits you blow up.
Perhaps add more of a purpose behind the lightning/energy than point increases? Maybe your ship's batteries need outside powering and you have to capture those energy cells to stay aloft? Perhaps each energy cell gives a boost to your speed or your rate of fire?
I think the previous mention of adding a real soundtrack to the game is a mentionable one. Many games get great reviews on excellent soundtrack alone ... you've got more going for you, but it's a start!
Great start, don't stop now .... but I'd definitely recommend doing some upgrades to what you've put together so far, just to make it that much better.