Finished it just now, quite fun.
I agree with the others that the furnace could need some more upgrades in addition to of course more ores eventually. What i found out though (i don't think it's intended) once you have a good equipment, you can go to a lower level and just spam downwards which will result in very easy level ups. Seems like the XP doesn't scale with the level you are on and is only based on the blocks destroyed? Also you drop through to your elevator once you reach the bottom :)