Easily the best game this year
Some people tout this game as being a mere extension of the first game. In many ways they are right, but in many more ways, they are dead fucking wrong.
Borderlands 1 was a surprise hit, taking Gearbox to a new level of game design. It was a FPS based around loot - an uncommon concept that was executed in a moderately good way. There was a great deal of variety in the randomized weapons, but a lot of them were very similar to other ones. The randomization system was impressive and lackluster all at once. You'd get something interesting every once in a while, but it was mostly a lot of the same. You were wow-ed at first, but the novelty ran out when you were level thirty. Additionally, the characters and world felt fairly bland and one-dimensional. NPCs were static, simple caricatures. The world was pretty much a dead desert. The enemies were predictable and easy to dispatch after a very basic pattern was learned. The bosses were kind of neat, but a lot of them were "Bandit A with more health, a name, and a neat weapon. Alternatively, they were Monster B but bigger.
Borderlands 2, however, destroys the precedents and makes the first game look as simple as it really was. The weapons are incredibly interesting through and through. There is such a diversity in weapons that sometimes, you just don't know what to do with all your crazy, awesome shit! Always something new and interesting hiding behind flavor text. Sometimes there's no hint until you throw your weapon as a grenade... except it homes in, or acts like a boomerang. Every time you experience a new combination, you get that "Whoa!" feeling all over again.
The atmosphere has changed a lot. The environments are beautiful, the music is always fantastic and fits the areas just right. There is more color! It's like Gearbox remembered that orange and brown aren't the only colors you can use! Playing this on an i5 + 6970 HD nets me 60 fps consistently with maxed settings. The game looks utterly fantastic in 1080p, and the environments really can look breath taking!
Enemy AI is much smarter. They dodge, jump, flee, and attack at all the right times. The enemies are more diverse. This means you never quite know what to expect from your enemy. Loot Goons can run around with a loot chest on their back (or a slot machine!), they can also leap 100 feet into the air to smack you for most of your health and call you names. Enemies in some areas level up as you progress the main story line, so you might come through an area and find a Super-Bad-Ass where a regular ol' grunt used to roam. There are several new RPG elements like that that were added carefully and with great skill and success.
Overall, this game is a massive improvement over a game I already enjoyed. Everything is better, (including quests), and it's just a real joy to play.