From Wikipedia:
Halo 3 will utilize advanced graphics technologies, as detailed on Bungie Studios website.[41] In the weekly updates on the game, "Frankie" tells of physics enabled vegetation, real-time shadowing and reflections, a new way of making the sky, distant gunfire sounds, photo-realistic moving water which will go around objects in its path, a wind effect which has an effect on the player, grass that sways in the breeze, and that all enemies, comrades, objects, vegetation, and the player will all cast shadows.
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Bungie also stated that they have redesigned the UI (user interface) from Halo 2, apparently making it easier to generate a custom game in Halo 3 multiplayer.[46]
Hosts will now be able to "advertise" their custom games via Xbox Live Public; players no longer need to adhere to pre-made Matchmaking playlists because they can simply join a custom game hosted by another player. [42]
When a player is carrying a second weapon in Halo 3, the secondary weapon will be stored on the character's back or leg holster, depending on the size of the weapon.[31]
The game will feature the ability to record and send video clips via Xbox Live at any time, making Halo machinima even easier for users.[15]
In the November 11 Bungie Weekly Update, it has been stated that due to a "significant chunk" of Halo 2 players being female, there might be an option to have a female voice for the Spartan character in multiplayer.[47]
"As it stands", the strength of the effect of gravity is customizable as one of the game settings in creating custom games.[49]
A new feature dubbed the "A-hole button" will be used to mute players in the game.[50]