As others have mentioned, this game would be perfect if lifting your finger didn't drop the current piece. If I am dragging a piece side to side and want to reposition my finger, or even click pause, it drops the piece. Instead, I think a second gesture like a distinct tap should be used to place, or even a button off to the side. This is not a fast-paced game like Tetris; it is thoughtful and calm, so there is no reason for hair-trigger controls. If anything, they make the game less calm and incite screaming. I can only imagine how frustrating this must be for people who have trouble with their hands.
Another design issue, this one minor: the view is semi-3D, so you see the top edge of the current row. However, the rest of the pieces below that row do not really appear 3D, and you see only their face. This makes the top row look blurry and misaligned with the background grid (once I saw it, I can't un-see it). I really like the rest of the design aesthetic, but I don't think there's any reason for the pieces to be shown this way. Purely face-on would be less distracting.