Ooo, this is a good one. Usually not huge on random tetris variants, but this one really clicked (especially when I realized that you clear rings. Being able to move around gives the game a lot of depth and makes it a lot more engaging to play. The game also has a really nice difficulty curve. It starts off quite slow (which originally bothered me, but it was fine once I understood ring clearing) but gets hard fast.
One suggestion I have would be to have two buttons each for rotating the center tile and the incoming tetrominos. There are a lot of times I'd press X to rotate the center tile in the other direction and end up rotating the tetrominos instead, or vice versa. If we're keeping arrow keys for movement, maybe map Z and X to rotating center piece and A and D to rotating the falling pieces.
I also think the music would benefit from a little more energy. The classic tetris theme has has a lot more gusto to it; would like to see something similar here.
Great game all around! Didn't expect to like this one so much