This wasn't just a sequel, it was a kiss-and-make-up meta-equal for the first.
Remember How "Highlander" was this awesome amazing movie with a perfect story and a definitive ending and then they did a horrible sequel that actually detracted from the original movie?
Now imagine that someone, some entity, some group of code-monkeys who could write both for Saturday Night Live and for the Department of Defense figured out how to turn an original win into a sequential OMFG Halleluja! in exactly the opposite form.
They did that.
BL2 gave greater depth to the original story in ways that I can't describe without making spoilers.
My advice to anyone who loves RPG's or FPS's is to find the first Borderlands for their system, go through it, meet the slight disappointment all of us did, and then PLAY BL2.