Overall, Astrocube is a very good game and I find it easy to pick up but hard to put down. The title screen is amazing as well as the game's graphics and animations. What's great about this game is that at first, the player would have no idea what to do, but quickly learns and builds up their skill at the game. Haven't tried two player mode but I'll make sure to sometime.
Apart from that, a tutorial level would probably be good, unless you have anything against that, and in that case, don't add one. Just saying though, it would probably be good.
Also, There's a particular screen/level that you respawn at when you die. That confused me a bit. Maybe adding a checkpoint whenever you enter or exit a screen would be a good idea. (there's some great tutorials on the forum!)
And in that level, there's a rhombus-shaped collectible on the lower-right side of the screen that kills you. hmm...
But apart from that, Astrocube is a wonderful game. I can't wait to try it out when the full version is released!
:]