This has amazing potential, but because of my limited artistic creativity, I feel a bit limited by the pieces I'm given. There are top and bottom hemispheres, but you don't have bottom curved pieces (so I can't even make something very basic like say, a wheel...) or bottom pyramidal pieces. It would also be nice to have finer layer control for the thinner pieces, in case you wanted to make something two-thirds the thickness of a thick block.
It's a cool "sandbox" toy though, and I hope you develop it further.
Oh, and since this is Newgrounds... I'm surprised nobody has done this yet:
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(maybe they have, but it's just on another page of comments)