Xbox.com's weekly newsletter points out an interview with Frankie about Halo 3. He says the focus in Halo 3 is playing it your way:

I hope that the main thing players take away is how much Halo 3 is their game. That is to say, it's a game that lets them play it the way they want to, whether that means joining three friends for co-op, or hooking up with eight friends to build a new Multiplayer variant in Forge, or simply share screenshots and movies with their own subset of the Xbox LIVE(R) community. We really have gone all out to make sure this game has more initial impact, and way more replay value than any other Bungie title to date.

There's also something about looking for two (or is it three?) hidden screenshots that littlebigman pointed out in the HBO forum.