I have never played a Halo game. Of the last few generations of consoles, the only one I haven't owned is the Xbox. I live in Japan, so there. Who better then to go attend a private session with the game? (I really am excited about playing Halo 3. Really. I'm pre-ordering tonight.)
Heading up the show were Jonty Barnes, head of production at Bungie, and Jay Weinland, "Sound Monkey" (or if you prefer, sound designer), the whole proceedings taped for Gametrailers.com (you may see a few questions from me). There wasn't anything new that most readers here, reservation ticket in hand, don't probably know. Oh, I know how it ends, but you'll just have to wait to find out for yourself.
A few things though. Downloadable content from Bungie? They'll wait for a while, see how the player-generated content created through Forge works out, but expect some official downloads down the line. Easter eggs and stuff to do after you've gone through the storymode once? Play co-op, as it opens all sorts of new interactions with the AI, and then hunt for the skulls — collecting them will open up plenty of new hidden modes. Keep looking for terminals as well, since a separate back story is told that way.
The Forge functionality really is something, and the idea of players taking on the role of a 'god', adding/removing stuff from maps on the fly as games are being played — you can even score points when in edit mode, dropping tanks from the sky on unsuspecting players — is going to change the way multiplayer is experienced. Add to that the possibility of inventing new sorts of gameplay — the developers were showing off a game of baseball with rockets — and you have the recipe for some seriously creative fun.
As far as follow-ups? Mum's the word, although they are really happy about the way things are going with the Peter Jackson project, and details on the game will probably be announced early next year.