Thanks for the feedback!
I think it's possible to add WASD to the web version, I'll give that a try once the rating period is up. I was thinking about having the shields stack, but I wasn't sure if that would make it too easy to accumulate a lot of shields early... Maybe I'll try letting you have a second layer at least. It's not really obvious, but if you pick up a shield powerup while you already have a shield, you get 5 points, so it's still worth grabbing them.
Yeah the asteroid impacts are cool! One of those things that ended up being simple to add, but made a huge difference.