Game Informer are doing one of their big, slow reveals for Borderlands 2, and some details have already begun to trickle out. The characters from the first game are apparently now NPCs in this game, and robo-host Claptrap will also be making an appearance. The new game will have a currency and resource system, based on an element called "Eridium", and there will apparently be a big overhaul of the weapons. These will now be customisable and more visually distinct. There's also going to be wider range of vehicles.
The biggest improvement, however, seems to be in the NPCs, who will be more dynamic in combat, interacting with each other and so on, and also moving about and interacting with characters in the world, which they entirely failed to do in the first game. It's looking intriguing.