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IGN has some brief snippets on Borderlands 2:
More information.Another change is inclusion of dynamic story missions, in which the story will shape to fit your success or failure.
A new element, Eridium, will also be present. It's not a weapon, but can be combined with guns and vehicles to improve their stats. And it's also used as a form of currency.
Enemies also promise to be more varied and more intelligent.
If you got attached to a particular weapon in the first game, it's time to say a tearful goodbye. Gearbox is scrapping all the weapons from the first game in favour of a new-look arsenal.