In Tetropolis, you play as a defective tetronimo. As the story goes, there's a factory that creates all those puzzle pieces for all the puzzle games. Perhaps that's why you only see the next one piece in Tetris-- it's the one currently being made! At any rate, our hero gets made with only three blocks instead of four, and is sent to the disassembler. Along the way you'll meet another defective tetronimo, one with only a single block, and work together to try to escape disassembly and find a place to fit in.
The gameplay is what's really intriguing here, because while there's some standard platformer fare, there are also some puzzle sections where you and your cohort will have to reassemble yourselves into a specific piece to get into certain areas. This leads to some Metroidvania-style backtracking, but what's more, all the rooms in the factory are also tetronimoes, and you can rearrange the rooms to access parts of the map that are normally off-limits. The lead developer of the game calls this "Tetroidvania."
Joystiq was at GDC and shared their impressions here: http://www.joystiq.com/2014/03/21/tetropolis-the-platformer-starring-tetris-pieces/?ncid=rss_truncated
And you can read about the game on the game's official site: http://tetropolis-game.com/