Things Learned From Gears of War 2 Gameplay Trailer
The Locust now have the ability to sink entire cities with giant emergence holes.
To defend against the Locust onslaught, the COG transport vehicles called Derricks bring tunnel machines called Grind Lifts to Locust occupied areas. Underground combat ensues.

Campaign level act one, chapter three is named "Assault".
Reavers are back for GoW2 and they are trouble.
Both the cover system and active reload look to stay true to their Gears roots.
Overall combat is much more open and expansive, it truly looks like you're in the middle of a war.

Brumaks are back and yes, you will be gunning them down. Take that Gears of War PC!
The ability to generate hundreds of Locusts on screen looks (as we expected) to be used to set the mood of the battle and make it more frantic. Think of these hundreds of Locusts as interactive decoration.
We're introduced to a new Lancer style weapon that shoots ammo in bursts.
We're also introduced to a new pistol weapon that sort of looks like Halo 3's Brute Spiker.
When a Locust falls to his knees you'll be able to use him as hostage cover.
Locust neck snapping is now an option.
They've added a new chainsaw animation that was seen in the teaser trailer. Basically, the chainsaw is flipped over, stuck into a Locust's back and is forced up vertically through the torso. Bloody disgusting.
Chainsaw duels are real! The chainsaw victor is the one who can press "B" the fastest in the mini game.

Assault Derricks pack a lot of firepower and do a pretty good job at running over Brumaks.
Gears of War 2 is going to be incredible!