First of all, the visuals and sound are fantastic. Really eye/ear catching, and both compliment one another nicely.
The gameplay was a little dull at first, and I worry that someone with a small attention span might dismiss it as simply collect minerals - perhaps putting just one or two enemy ships in the early stages of the game would ward against this?
I didn't read any of the text. There ought to not be bubbles of instruction text popping up infront of the player; save that for the menus. I think some quick, wordless, and easy to interpret diagrams would do wonders for the learning curve, brief though it is.
I didn't discover any way to upgrade weapons using only the keyboard. If there isn't one, perhaps the number keys 1 through 6 would suffice, along with a descriptive label for the upgrades - Guns, Missiles, Mines, Warp, Engines.
One more thing, it took me a while to figure out that my cargo space was full, and that I had to offload the cargo. A warning graphic, and a different pickup sound, when the cargo space is full would help new players avoid making this mistake.
Overall, it's a really solid and fun game - keep up the good work