Its coming really soon. The scheduled release date for the full game is September 14, 2010. You can pre-order the game already from many places. Its not that different from Halo 3 really, just some choices for what type of character you want in the multi-player modes and each has some special abilities like super-speed, jet packs, invisibility, or you can even be one of the alien Elites in some modes. New levels finally for multi-player too finally.
I am hoping the game designers will include the option to play all the classic Halo 3 maps with the new abilities. But since its a Microsoft game, I don’t really expect them to be that creative. There are so many more elements they could have added to Halo to give it more variety in both the multi-player and the campaign modes. I keep imagining if they combined the Halo shooter game play with the Fable RPG style system of collecting items, being able to build your character up, and evolving game enviroments. That would be the ultimate game!