Amazing game and wonderful work! I've spent a long time playing this already and have explored all the biomes, though maybe not all of the terrain itself.
There were only two things that I disliked about it at all. The music didn't take long to get repetitive, but I'm sure that's why you added a "mute" option. Thank you for that!
Also, I found some problems with collision where I would get stuck trying to fit through a small gap (between trees, or on the ramp areas between terrain levels).
A coupe of things I noticed: I was able to get the diamond shield right after I got the hammer, since pools of lava can be destroyed with the sword.
Also, the blue and yellow slimes were immune to both the diamond and steel swords for me, so I had to switch to wooden again whenever I wanted to kill one. Those last two things didn't detract from my enjoyment of the game at all though.
The pixel art is great, especially the variation on the biomes! I loved the sound effects, too - would have given a higher audio rating if not for the music.
Overall I loved the feel of it and was very impressed with the scope. Awesome job! :D