On the game play side, it's a bit boring after a while. Of course you're still developing, so this is but constructive criticism. It definitely needs more depth, like power-ups and enemies. On the graphics side, I'm usually never picky because I'm a terrible artist, but I find that immersion suffers from the coins. Maybe if they could be replaced by metal ores it would be more coherent with the game's world.
Other than that, great job!