Some of new features include two new weapons: a Spike Rifle and a Spike Grenade, and the appearance of an ATV vehicle called the Mongoose which was cut from Halo 2. You'll also now be able to record your multiplayer and singleplayer games and rewatch them.
Another revelation is that three versions of Halo 3 will be available when the game ships. There will be a regular no-frills version of the game, a collector's edition, and a Legendary edition which comes with a replica of Master Chief's helmet and includes special content spread over four DVDs. More details are in the December issue of EGM which should be reaching subscribers soon.