I liked the 3d backgrounds, and I thought the multi-lingual warning text was a neat touch (even though I don't understand the other languages...). The game looks great on my computer (i7, dedicated GPU). Slowing the game when the cursor moved out of the play area was a nice touch. So were the enemy paths, although getting a nice obvious bonus when you shoot down an entire wave would be nice. I don't know if it's your idea or you got it somewhere else, but setting the difficulty by allowing different upgrade paths is a really cool alternative to the usual selection of "easy, normal and hard" at the beginning of the game (but see below).
On the small negative side, better contrast between your bullets, enemy bullets, and the enemy path lines would be nice (maybe difficult to pull off though). Also make it more obvious that one upgrade path is "easy" and the other "hard" (I missed the small text at first, and it's not immediately obvious that once you start down one path you can't go down the other). And having no more upgrades for the remaining few levels is sort of a downer. Finally, not being able to move up and down in the first level is disorientating. It's over fast enough, but I worry that some people might be so irritated that they would quit before they find out it's temporary.
Great job!