This is without a doubt my favorite game on the citadel in my library.

Story
The story in itself is rather basic, you're on an alien planet, looking for legendary riches. you get attacked by a company trying to keep those riches for themselves. you befriend a group of rag-tag mercenaries (PC's from Borderlands 1), who help you defeat the company together and shoot the CEO in the face. Drinks all around!
so if you're someone who likes a game with intriguing plot-line with lots of twists and turns, this game might not be for you.

However, if you can see past the slight lack of depth in the story, you will find a world filled with interesting characters, cleverly hidden Easter eggs and a healthy dose of humour.

Difficulty
The game has no difficulty setting, but the first playthrough is easy enough to finish without too much of a hassle, which caps the enemies at 30-38, depending on DLC/base game.if you do all the quests, or even just the ones you like, you should be able to breeze through the enemies rather quickly (barring some bosses, mind you.)

The later playthroughs get progressively harder, as basic enemies will start to gain levels faster than you do, sometimes even higher than your own level cap, which was raised to 80 by the latest dlc.

Graphics / Style
I'm going to be honest here, when I first started playing the borderlands series, I was a bit taken aback by its chosen style. But as time went on and I poured hour after hour in the games without realizing it, it kind of grew on me, as I started to like the clunky animations and the comic-style shading.

DLC's
Because the game has quite a bit of DLC's available, I won't go into too much depth on each individual dlc's

• Mechromancer pack adds a new playable character to the game, a young robitic expert named Gaige. Gaige's talent trees can help upgrade her robo-buddy, enhance electrical damage, or say “f*** logic, I want to aim like a drunk that just came of a tilt-a-whirl”. This DLC is one I fully recommend.
  
• Psycho Pack adds another playable character to the game, this time it's a psycho named Krieg. Krieg's talent trees focus on making your enemies bleed, enhancing your Buzz Axe Rampage special skill (which is a sight to behold), or lighting yourself on fire so you can light your enemies on fire! It's a psycho, what did you expect ?
  
• Captain Scarlet and her Pirate's Booty is the first story DLC for Borderlands 2, and sends the players to a unusual sea where a legendary treasure lies waiting for them. Includes a new nautical themed skin and head for each playable character.
  
• Mr. Torgue's Campaign of Carnage is the second story DLC for Borderlands 2. After Mr. Torgue finds a vault that'll only open for the “Ultimate Badass” the players get an invite to partake in his new tournament. This campaign contains EXPLOSIONS, EXCESSIVE USE OF CAPITAL LETTERS AND MORE EXPLOSIONS!!!
  
• Sir Hammerlock's Big Game Hunt begins with Sir Alistair Hammerlock inviting the players along for a sport hunt on the continent of Aegrus. Uncover the most exotic creatures Pandora has to offer, and give 'em the old one-two!
  
• Tiny Tina's Assault on Dragon Keep is what would happen if you take borderlands 2 and Dungeons and Dragons, put them in a blender and hit puree. Filled to the brim with references to all manner of fantasy films (including a few cooking methods for po-ta-toes) and series.
  
• Commander Lilith & the Fight for Sanctuary takes place after the events of “Tales From the Borderlands” and features the players defending Sanctuary against a new enemy. As usual this DLC comes with new heads and skins to unlock.
  
• Headhunter DLC's these five “short but sweet” dlc's each contain a questline based on a seasonal event, Halloween for “T.K. Baha's Bloody Harvest, Thanksgiving for “The Horrible Hunger of the Ravenous Wattle Gobbler”, Christmas for “How Marcus Saved Mercenary Day”, Valentine's Day for “Mad Moxxi and the Wedding Day Massacre” and finally summer in general for “Sir Hammerlock vs. the Son of Crawmerax”.each DLC has it's own story and head/skins.
  
• Skin/Head Packs : in total there are 30 skin packs for sale, each adding a skin and head for one of the player characters. For $0.99 each, I'd say these are the only DLC's that are hardly worth getting, as the changes are purely cosmetic and I personally liked the story DLC skins and heads more. Get them with a GOTY or the Handsome Collection, but otherwise negligible.
  
• Ultimate Vault Hunter Upgrade Pack 1 & 2 basically increases the level cap to 61 and 72 respectivly. Upgrade Pack 2 also adds the Digistruct Peak challenge, and the Overpower ranking system. I highly suggest getting this DLC if you haven't already.
  
• Creature Slaughterdome adds a new arena for players to conquer, consisting of 5 rounds of increasingly tough animals. Not really what I would call essential, but fun nonetheless.

Replay Value
As I said in the difficulty section, the game gets progressively more difficult with each playthrough with the same character. This makes consecutive playthroughs worth the effort due to you still advancing your character. Also playing a different character, or with other players further enriches the Replay Value of the game. The last thing that comes to mind when talking about replaying borderlands 2 is the Badass System, which rewards players with Badass Tokens for a plethora of challenges, like finding all Vault symbols in an area, or killing 3200 enemies with an assault rifle while crouched.

Sufice to say, borderlands 2 has a ton of things to do before you're really done with a character, and even then, there's five more characters to play, each with an entirely new skillset and collectible skins/heads, making this game one of the greatest games in terms of replayability.

Conclusion

Borderlands 2 is one of (if not the[/]) greatest co-op first person shooter to date, with memorable characters, a bit simple storytelling (which has its own charm) and most of all, hours upon hours of axe-slinging, gun-shooting, car-driving, head-shotting, 'nade tossing fun for you and up to 3 friends.

If you haven't bought the game yet, you should.