You could try giving it some form!  You have a clear light source but its effect on your shapes is hardly seen.  You could also try pushing the design more, less mario ripoff and make it into something unique?

1 - TOO BAD.  You need to redraw his whole body.  He's got wee little midget legs!  Also, he's still pillowed shaded and mirrored all to heck.  Pillow shading and mirroring are pretty strong signs that you might enjoy working with a smaller image, one that doesn't take so long to shade and doesn't require so much detail.  I've made very few pieces that are the size of your knight!

By disregarding useful color theory your end result is flat, confusing and uninteresting, despite the fact that by simply applying some basic graphic design ideas and not changing the actual tile designs you have something that is really fantastic.

Part of challenging established conventions is realizing that SOME conventions aren't done just because they're easier, they're done because they work better than everything else.  People have been doing art for a pretty long time now!  I think you've chosen some very strange things to re-invent.

Also, I don't see how applying MORE limitations in this already cramped space makes the design work any easier.  I think it's much harder to come up with some cool, original tilework that would actually be playable AND fits the hardware restrictions.  It's easy to make something "different" when you don't have to worry about whether or not you would alienate anyone who tried to play the game.

Last 3 posts are all great crits, I think if you ease up on the shadows, go back to orange and blue, and stick to crystalline shapes you will have something really, really awesome on your hands (its already pretty sweet) - great work so far!