Braid is a game about time manipulation, and the human condition.
"What if you could undo mistakes?"
The game explores this question heavily in both story and gameplay. You play as Tim, a character that is off in search of the princess (there are a TON of homages to old school video games in this game). However, this is because Tim made a mistake, and caused the Princess's braid to turn sharply away from him.
You also have an ability that is unseen in most games... full and total control of time, from all the way to the beginning level to all the way to the present. Each new world you enter re-writes these rules of time. World two will have you just using time to fix mistakes, but world four will make it so you position on the world affects the flow of time (If you move forward, time moves forward, if you move back, time moves back... and your rewind+fast forward ability still works).
At it's roots it's a puzzle game with every level hand-painted, and a little bit of platforming, but Braid is also renowned for it's amazingly deep story for such a game, that teaches us about ourselves as humans.
As the highest rated game on Xbox Live Arcade, and the 11th best game on Xbox 360 of all time for a 2D platformer/puzzle, I think you owe it it yourself to atleast try the demo out on Steam and maybe even purchase it.
Braid is available on Steam (and other digital distribution outlets) in right now, at a price point of $15, and is one of best games I've ever played on the 360. You can download the demo (and a non-steam version) here: http://playgreenhouse.com/game/NNONE-000001-01/