When an original concept is created for a video game and it is made into a new video game, it is difficult to see how early decisions well end up in the final project and generally you don't have the budget to go back and change decisions you regret. Furthermore, the budget is usually very tight so you end up cutting features. The entire project usually ends up severely lacking polish. This does not mean that the game is bad, only that they rarely live up to their potential, especially when the idea is new and radical.
Borderlands the first, being such an ambitious project, had these problems in spades. The game lacked variety in environments and enemies, the combat was imbalanced, it started slow, and the combat wasn't that satisfying. Still, I greatly enjoyed the time I spent with it. The idea of a true FPS RPG loot fest was excellent. Finding a new shiny gun was great and playing co-op was an excellent experience.
Borderlands 2 seems to have nailed down most of the problems that plagued Borderlands the first. The combat is fun and satisfying, the enemies and environment show much more variety, it is very enjoyable from the very beginning, and even the humor and randomly generated equipment is better. Borderlands 2 is not a perfect game, and I still need to play it a great deal before I can make a final judgement, but I think it is a good game and a great example of how sequels are important in the video game industry.
That is not to say that all sequels are better than the original or that too many sequels can't be a problem, but that is a subject for another time.