One thing I might suggest (and I don't suggest this for every game), is Touch controls.
I realize that not every game can be a Mobile game, but I feel this game in particular might work well as a Mobile game, in addition to being a Desktop game.
As for the question of the difficulty curve being too gentle - I don't think it's too gentle. I see this as a fun game that leans toward being challenging.
Overall, I like the game and the graphics.