Cover shooters, unlike most first-person shooters, require a tight focus on defensive positioning, for one thing.
There’s still running and gunning aplenty, but you’re often holed up or barricaded behind some protective structure. What one loses in accuracy and immersion, one gains in a new layer of tactical gameplay.
In third-person melee can play a much larger role as well, and in Borderlands 2 it could play a far more integral role in the game.
Melee combat could more easily become a dominant feature of each character class, opening up a whole swath of hand-to-hand weaponry on top of the bazillion guns, not to mention a much deeper melee skill tree.
Depending on how the camera worked, the third person perspective could give you anywhere from an over-the-shoulder view to a bird’s eye view, fundamentally altering the landscape of combat.
Finally, third-person would mean you could see your character all the time (except for when aiming through a sniper rifle or a pair of binoculars) rather than just after dying or in the inventory screen. This would place a higher priority on customization, another key element of any good RPG.