Really nice work!
Runs ok on my old laptop (2.2 GHz, 3 GB RAM, GeForce 8600M GT). When adding hills and caves it gets somewhat laggy.

Perhaps you can even implement Minecraft's lighting style? I don't know how to do that, though. According to Notch "it adds no performance penalty per frame, and a small one every time a tile changes". Perhaps it's a bit tricky.