Instead of sitting around in the lobby waiting for a match to be found, you can now play some other games while matchmaking is running in the background. If you decide you want to play multiplayer, the game will most likely already be found, and pair you with people similar to you now with the new reputation system.
Via VentureBeat
"Today, typically what happens on Xbox 360 is you go into matchmaking, and you’re sitting around in a lobby waiting — you’re sitting and not playing a game,” said Whitten. “The question is why can’t you be running the matches that you like — that are really tailored to you — always in the background so that whenever you’re ready there is a match that is ready for you. There are always more matches.”
Via VentureBeat