Map voting has been revamped this time around as well. In Halo 3 players were given the option to veto any given map in online matchmaking. If the majority of players voted to veto, a new map was selected and that was that. The problem there, according to Bungie, was that you might just wind up with another map you don't like. To rectify the problem, Halo: Reach presents players with a default map and three alternative maps in matchmaking. If players don't like the default choice, whichever alternative map receives the most votes will be chosen for the next round.
The other major discussion was the new Active Roster system. Essentially, the Active Roster takes all of the tediousness out of finding friends to play with. Rather than forcing you to open the Guide and find out what your friends are playing, all the information is displayed right inside the matchmaking lobby. From here, you can see what your friends are doing and create or join parties. Furthermore, Reach has a new "queue join" which takes the hassle out of waiting for your friends to finish matches.
"Queue joins are a stab at solving the thing that was most irritating about trying to get into a game with your friends in Halo 3," says Bungie's Tyson Green, "waiting in the lobby, sending messages, and hoping they remember to invite you between games." Now, if you attempt to join a friend's game that's already in progress, the system will wait for the game to finish and then automatically join said friend's party.
All in all, the update has more info than we could condense here -- including some tidbits on the social settings illustrated above -- so you'll just have to head to Bungie.net and read it yourself.