I like the concept quite a bit, had some fun with it.
I think one thing that made getting points hard was that sometimes you just don't get the right sequence of blocks and there's no way to move them closer together. One thing that would help a bit with that is removing the one square blocks. You could also increase the spawn rate, and maybe slightly decrease the falling speed to allow more chances for getting points. This might also allow the player to get more than two pieces cleared at once.
Another idea would be to have the gun move/rotate the pieces, though that's getting into the territory of an entirely different game.