Destruction -- "Most of Mars could be destroyed in Guerrilla, but the areas for destruction spread out in pockets. Our goal in Armageddon is to have destruction all around you, all the time. Moving part of the story underground gives us the opportunity to place destructible buildings and structures on cave walls and ceilings, which helps bring the caves to life."
Joystiq addendum: The change also eases some of the framerate issues caused by expansive, open environments.
Nano Forge -- "We introduced backpacks in Guerrilla multiplayer as a way for players to augment themselves in combat. With the Rhino ability they could run straight through a building and bring it down on itself, an experience that wasn't available in any other game. In Armageddon, we want to make sure the player has these types of abilities, this time through the entire game, so we re-created the Nano Forge. Now the player can use forge abilities in combination with weapons to create maximum destruction. Try Shockwave to freeze a group of enemies in the air, then switch over to the Magnet Gun and grab a chunk of building to send hurling at them while they are defenseless."
Joystiq addendum: Constructing a concrete barrier doesn't really get fun until you squash an alien with it. It's about as satisfying as taking down a building with the hammer in Guerrilla.
Storytelling -- "In open world games the player may spend so much time exploring the world that they lose track of the main story line. Guerrilla was forced to tell a much looser story to fit this mold and it was hard for the player to sometimes feel immersed in the world. In Armageddon, we want to focus on telling a gripping story ... By structuring game progress, we could ensure exactly when each story element is presented to the player and pace events for a truly emotional experience."
Joystiq addendum: GTA: Mars is out. Structured experiences are in, leading us to ...
Aliens -- "We've only shown a couple of our basic enemy types so far, but we have some much more exotic enemies come that will drastically change how the player approaches each area. Now that the you are in a cave environment with opportunities for destruction at every angle, we wanted enemies to come at you from every angle too. This takes the gameplay experience to a whole new level."
Joystiq addendum: We've also been told that the "spawn well" issue of Guerrilla, which generated enemies wherever the player wasn't looking at the time, has been addressed. This led to never-ending waves of baddies interrupting the flow of the action and coming from areas that were just cleared out.