One of the most prevalent changes players will notice in Borderlands 2 is the skill-tree functionality in co-op mode. Instead of simply beating down enemies separately but at the same time, as often happened in Borderlands, there will be more synergy between each character's abilities, Kester and Burch said.
For example, the Siren's phaselock abilities have been fine-tuned to include the "sweet release" kill skill, which insta-heals every member of her party if she kills an enemy frozen in phaselock. Additionally, the Assassin can execute enemies in the Siren's phaselock relatively easily with his melee or sniping moves, and the Commando will be able to throw down a bubble shield to protect the entire team. The Gunzerker is nuanced as well, including a skill that ramps up the power in the last shot of his gun's magazine, and if an enemy is killed with an excess amount of force, the extra points are converted to health. He can also dual-wield any weapon, which is awesome for everyone. Except the guys he's shooting at.