Halo 3: ODST
I started playing the campaign yesterday. It follows two types of missions. Day missions, in which you play as a member of ODST. These missions are exactly like those you've played in previous iterations, a linear series of fire fights, occasionally escorted by walking fodder.
The night missions are the other. During these you play as a faceless character known is the rookie. These missions actually are a little bit different. For the most part, the rookie seems to be obsessed with going back and forth through a quarantined city. These hikes can follow a number of routes though! You can choose to take on two hunters while being shot at with a carbine or you can take down a gaggle of brutes and grunts. The choice is yours!
One new feature is the visor. They're glorified night-vision goggles that highlight foes and otherwise. It's essential for the night missions as you cannot even see your gun in front of you without it. One nice thing is it doesn't seem the covenant forces see well at night. You can pick them off at a distance, while they swing around trying to find you.
As far as weapons go, I've seen one new one, a silenced SMG. Unlike the previous SMG, this one can hit the broad side of a barn. The recoil is more manageable as well. Those who played Halo 1 will be happy to know the pistol is back. They will be disappointed in its stopping power however.
I've finished it and there are not any surprises. Same enemies, but no elites. Same weapons for the most part, no dual wield. There is no matchmaking for Firefight, if you haven't heard already also.