I love the way they implemented multiplayer custom games in SC2. In previous Blizz games, if you wanted to try a custom map, you would get booted from a room the second you joined because they would see you downloading it. People playing them tended to get insular and didn't like anyone new joining in on the fun.
Now you can still host a game and let only friends join if you want, but if you want a public game, you select a popular one from a list and get put in a room nobody has control over and the game starts when it is full. It's worked great, so far. People are a lot nicer, too, because you get a lot more other people who haven't already mastered it to the point where it's impossible to learn.
I even played a 1-player game on there called "The Hungry Baneling." It's Katamari Damacy set in Starcraft. Hilarious. Your baneling gets huge! I mean REALLY huge. Normally large units and even buildings look like ants! The scaling in SC2 is incredible.