I think that for fast play, a better texture than rough metallic could be used, as it looks too similar to solid. Perhaps a checkerboard or striped texture, something that's "halfway in between" solid and transparent, that would contrast better with both.
To stretch the spatial thinking further, you should create a harder mode, in which players can rotate the numbers to one of 4 orthogonal orientations (right side up, upside down, lying on left side, lying on right side), and require that numbers on opposite sides of the cubes have the parallel orientations (both have the same "up" direction) in space.
People are complaining that the game must cheat because the computer got 3 straights in a row or 4 straights in a row or whatever.
The probability of getting a straight is 1/65. The probability of the computer (or you) getting 4 straights in a row is (1/65)^3 ~= 1 in 17.85 million. This game's been opened in browsers across the world 174,512 times. Suppose everyone played 5 games each time they opened Zilch in a browser, with an average of 15 turns per game. Then 4 straights in a row should have occurred (5*2*15*174,512/17.85 million) = ~ 1.43 times so far. Probabilistically, someone somewhere SHOULD HAVE rolled, or witnessed the computer rolling, 4 straights in a row.
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 "Zilch"){kind=icon}
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