I've been reading the Bungie FAQ, I am soo excited about Halo 2.
Some quotes from FAQ:
..new vehicles, destructible environments, new weapons, low gravity levels, online play, Human vs. Covenant multiplayer battles, improved AI, and more.
So far there have a been a few new weapons revealed. The Battle Rifle is an evolved version of the original assault rifle and features increased accuracy, a scope, and a slower rate of fire. The SMG (Sub Machine Gun) is a rapid-fire gun that trades a high volume of bullets for decreased accuracy. Of course there are others, but we can't tell you about them yet.
New enemies: The Brute has been described as a cross between a gorilla and a rhino and will be a serious force to reckon with on the battlefield. The Prophets have been referred to as "giant space popes" and they provide the political, religious and spiritual leadership in the Covenant. As for the others... you'll just have to wait and see.
This time around, the worlds of Halo 2 will feature destructible objects to add to the realism and strategy. Windows, columns and doors will shatter into pieces when they take damage. Enemy soliders will be able to recognize these objects and strategically destroy them to make you an easier target. In addition, MC will be able to shoot out lights and hide in the shadows when stealth makes more sense than confrontation.
The enemies in Halo were no slouches but this time around expect your opponents to be more cunning and tactical in their attempts to anihilate you. Covenant forces will use the environment to their advantage and make your job that much more difficult. You'll see the Covies scurrying under ledges, leaping over obstacles, and climbing up structures. They'll fluidly move with animal-like grace while talking and coordinating amongst their squads. They'll even use flashlights to hunt down MC in the darkness. It will take all of your wits and skills to overcome your adversaries this time around.
The Human forces will also be smarter this time around. They'll work better as a team and flip over objects to create cover. The Marines will also know how to call in support from a Warthog or point out an enemy sniper for an ally to take down.
Besides sporting an improved graphical look, MC will get his hands on a new suit of armor, some "tools", and a few new moves. Check out the 'Characters' section of the Halo 2 gallery for a glimpse of the Chief's new look. When things get hectic, the Chief can lower his gun and sprint at almost twice his normal speed. Before rushing into battle, MC can peek around corners and scope out the situation. When the action gets up close and personal, the Chief can string together melee attacks into devastating combos. The Chief is sure to have a few other surprises up his sleeve so stay tuned for future updates.
Besides a pumped up bump-mapping system, Halo 2 will really shine due to the inclusion of a new advanced real-time lighting engine. Dramatic lighting and shadows will behave realistically and interact with characters and environments. We've also got a new subtle technique called "self shadowing" where every model in the game will cast shadows on itself as well as its surroundings. Most of our new graphical effects are still in heavy development so we can't say too much about them yet. We can say that our goal is make sure that Halo 2 looks better than Halo in every way!
Thats most of it, but there is more. I am so excited about this..!!