- A good feature will be to place at the sides of board a pieces lost by players.
- Also you can improve animations of piece move (just like in vista chess for example).
- You can also add an antialiasing future to achieve smooth picture (and add an option in graphics submenu - for disabling it at slow computers).
- I am not a proffessional in chess game but I can say that AI plays not very good even in HARD mode, however thinks too long time. I am 3D game programmer, so I know the possibilities of modern computers - you just need to improve AI algorithm and it's speed. This is the main you should do in your game to make it popular (not models, skins for board, background and anything else you work at the moment).
P.S: don't feel hard of my comments - I really like what you do.