Since Xbox 360's launch, I've been trolling various message boards for feedback about my biggest feature: voice chat.  This post made the previous year of my life completely worth every minute.  There's something to be said about the power of Xbox Live to bring people together, regardless of what you think of the service.

That said, the most consistent piece of feedback is "Why can't I talk with multiple people in QuickChat?"  The simple answer is that it was a user experience choice.  The longer answer follows, but know that there isn't much in the lower level software that would stop us from letting users do this.  However, let's think about it some more...

Xbox Live requires users to have broadband connections that are at least 64kbps up & down.  On the other side of the coin, Xbox Live requires games to work within 64kbps.  Voice chat in a game uses some of that bandwidth.  How much depends on the number of users, mostly.  All Xbox Live games have voice chat in some way, so there is at least one user worth of "chat bandwidth."

The Xbox 360 guide is not omniscient.  It has a lot of info, but it can't know how many people are chatting inside a particular game.  However, it knows that there's at least one user worth of bandwidth around being used for chat because we wrote that rule.  When you use QuickChat, the Xbox 360 Guide mutes voice going to the game.  That means the game no longer has any voice chat to transmit, and hence stops using the voice chat bandwidth.  That frees up the bandwidth for QuickChat to use.

Imagine QuickChat did let you speak with multiple people at a time.  This means you'd need to send data to multiple people, thereby multiplying the amount of bandwidth you need.  This may make QuickChat go over the 64kbps total that is available.  If you have a much faster pipe, this would not be a problem.  However, measuring available bandwidth is a non-trivial problem that has had many academic papers written about it.

Going past the available bandwidth would make either your game or chat experience horrible - perhaps even both.  Hence, QuickChat doesn't go over the minimum requirements to make sure all our users have a consistent experience on Xbox Live.