Since I've got a bunch of PMs, I can elaborate here so you can all see.
We've tried over the past few months to more closely manage large communities. One of the original ideas behind the Community Forum was to ensure that large communities did not continually dominate the first page of the main board (among other things). Additionally, since relocated to the Community Forum, I think you'd agree that you've been pretty loosely moderated.
As a result, we've gotten very regular complaints from posters both long-time and new (or trying to break in) about the conduct of certain posters in the thread, the topics being discussed, posts violating TOS, etc. The issue is not "PMing moderators too much." That mindset is part of the reason why this was necessary in the first place -- because some people may think that these threads aren't moderated and they can get away with breaking the rules. Breaking the thread into two components-- people who want to talk about Halo and people who simply like Halo and want to talk with other people who like Halo-- makes it easier on us to moderate the thread as well.
This is, of course, an issue separate from our initiative to prevent exclusive communities from forming or becoming isolated and/or insulated communities. We believe breaking apart the thread will allow new members to break into the game-related discussion and will prevent the community from becoming too isolated/insulated.
The thread locking does not have anything to do with one member or one person's actions. Yes, there have been lots of non-Halo related things posted in here that get cross-posted elsewhere. That's a reason why we're doing this but not the only reason.
If you have suggestions, we're open to hearing them.