A lot of changes set Borderlands 2 apart from its predecessor, but not all of these changes are positive.
GAMEPLAY - The overall gameplay doesn't see much change here. You still run around shooting, looting, and completing quests. There are a lot of subtle changes to the formula though that really do add up and make this game feel quite a bit different from the first. The first change you'll probably notice is the HUD menu, which takes a little getting used to. Also, instead of leveling up each individual weapon type, you gain "badass points" from completing in-game achievements that don't count towards regular achievements/trophies. These range from killing certain enemies to picking up a bunch of hidden items scattered around a given area. As you obtain these points, you can upgrade stats such as gun damage, elemental chance, shields, and accuracy. We also see some minor changes such as revolvers now being rapid fire weapons, elemental weapons being more dangerous (not to mention a new elemental type), and a new resource called eridium which helps you increase the amount of items and ammo you carry.
There were some changes about this game I didn't like, however. For the most part, this game seems much more geared for a 4-player experience. Not that this is bad, but I do tend to enjoy playing solo usually. What ends up happening is that bosses are much more difficult than in the first game, with a couple of disappointing exceptions (I ended up sitting behind a rock and shooting the final boss until he died without taking much damage at all). The questing also feels a lot more tedious. Basically with every "chapter" you progress through of the main quest line, a new batch of quests open up. It's no longer dependent on your character level. Not only that, but a lot of quests involve you back-tracking to areas you haven't seen since the beginning of the game at times. I know there was the occasional back-tracking in the first game, but never this bad. I loved questing in the first game because you would get a slew of quests, go to an area, get them done, then go turn them all in. This time it feels a lot more like you're running back and forth trying to get everything done. Also, this game feels a lot more linear than the last in that you don't find certain weapons and certain mods to weapons/items until you've reached certain parts of the game. The first Borderlands did this with caustic weapons, but this game does it with just about everything. I've already completed the game and haven't found a single transfusion grenade mod, which was always the kind I used in the first game. Lastly, enemy AI is very spotty. At first you'll notice that it seems like a lot of enemies are a lot more intelligent than in the first game, but there are some enemies...including bosses...that just stand there and let you fill them full of bullets until they die.
Not everything that has been changed in this game works, but for the majority of people, everything is pretty positive. My main complaints lie with the questing more than anything else. - 7/10
STORY - Without revealing too much, this story is about you going to Pandora to find a second vault that has been discovered. However, you're up against another individual named Handsome Jack who's already a few steps ahead of you in searching for the vault. This game includes most of the characters from the first game, along with a few new ones. My personal favorite was probably the 13-year old explosives expert. They definitely did a better job with the story in this game, but it feels like the questing did suffer a bit as a result. - 8.5/10
GRAPHICS - Not much has changed here since the last game, but that isn't a bad thing at all. One thing I enjoyed was that there were a lot more environments outside of a desert to explore in this one, which let you see a lot more of what Pandora looks like. - 8.5/10
SOUND - Not much has changed here either. If anything, the dialogue is a lot more entertaining, but that's about it. - 8/10
LENGTH/REPLAY VALUE - This game is noticeably longer than the first game. If you want to just plow your way through it, you'll have a pretty tough time of it. There was one point where I thought the game was coming to an end, but then it just kept going. Don't expect this to be a short game, especially with DLC on the horizon. Between the four characters with two playthroughs a piece and the length of the game itself...this game will definitely keep people entertained for a long time. - 10/10
BOTTOM LINE - So while the things that make Borderlands 2 exactly what you'd want are there...good presentation, addictive gameplay, lots of replay value...there are just a lot of changes implemented that make a much more punishing experience for those who like to play solo. I don't usually play in a group, but I can imagine that this would probably not be nearly as difficult in that setting. Playing solo caused me to die constantly at certain parts, making this game a real pain for those like me. The questing has also been changed for the worse. There are some nice new changes implemented though, such as automatic pickups on certain items and crazier weapons. But on a personal level, I didn't enjoy this game as much as the first. It's not a bad game, by any means. It's just not quite as good as I was expecting when I went into it. - 8/10