If you don't know much about the game yet, go read an official preview. This thread is for detailed info.
Most information was taken from a PlayStation Q&A article, and a Gearbox employee who is currently posting on NeoGAF.
The game is just as long/longer than the first.
You’ll be able to choose from a large variety of swappable heads/hairstyles/headgear in addition to choosing from hundreds of unique color and decal themes, and both together offer a ton of options for customization. As for skills, each class offers a bigger quantity and more interesting variety of skills to choose from in the skill trees.
We haven't announced all the details yet but can confirm that some skins will be part of the loot system in BL2. Some will be super rare and drop from enemies/bosses etc.
If we detect a save file from the first game on your console then you’ll be rewarded with a unique swappable head and skin for every class!
We’re mixing it up a bit this time: few short cutscenes (and title cards) where necessary, lots of in-game moments, ECHO logs, missions, and environmental storytelling – not to mention the significant NPC improvements we made so it doesn’t just feel like a cardboard cut out of a dude is sending you on a quest.
What’s great is that we catered it towards both audiences that love story telling through NPCs and those who prefer to go kick some ass! If you choose to do the latter, you can run off as the NPC is talking to you and the discussion will appear as an ECHO transmission. No chains attached!
Inventory, pickups, and UI:
We have a completely new inventory screen and all of the other interface screens have received various upgrades. There’s a lot more useful data for gear comparison and also more descriptive text on the item cards as well. Having said that, the best way to know what a gun does is to use it – and some weapons are *very* special…
Money and ammo now automatically pick up if dropped from enemies as well!
You have to walk over [ammo] to actually grab it. It doesn't just fly from the enemies body to you. Also if you are full it won't grab it. Chests don't work the same way.. So if you find ammo better suited for your co-op partners in a chest, you can tell them that and they can go over there and physically grab it via Holding down the pickup button for all the loot or just one by one.
I would have specified a specific button for picking up loot but we now support full remapping of controls on all platforms. :)
Rocket Launchers are much better now. In fact all the guns are. The whole weapon system was revamped and they feel and look a ton better. Each manufacturer has a distinct look and they all have their own 'benefits'.
For Rocket Launchers, I would say Tediore ones are my favorite now. Tediore's benefit is that they have the fastest reload speed. This is achieved by you throwing your gun away when you run out of ammo or reload. The Tediore then digistructs a new gun into your hand.
When you throw your gun away it explodes and depending on how much ammo is left in your magazine is how much damage the explosion will do. So yes you can use it like a weapon. Rocket Launchers are big though... You don't really throw those.. Instead they turn into rockets themselves when you reload and launch out into the battlefield.
Health kit changes:
It was a tough decision, but we decided to remove carryable health packs and instead fill that need with new relics and shields that can help keep you out of that emergency state to begin with. The result is a more fluid combat experience since now you’re not worrying about when the best time to pause the game is and you still get that fun sense of preparing for a battle by choosing the right gear.
Health Packs will still appear and drop and they will now heal you up in the middle of battle.
We’re introducing a new vehicle, the Technical, which can hold 4 players at once and also has a few weapon configurations to choose from. Some are better than others depending on the types of enemies you’ll face and the environment you’ll be fighting in. On the other hand, Borderlands is mainly a game about weapons and loot so we don’t want to force people to stay in vehicles for too long.
There are brand new enemy families in Borderlands 2, and the total count of unique enemies (including special elemental variants with unique AI) is in the triple digits. New environments + new enemy types and behaviours = much more varied combat encounters!
The free pre-order Mechromancer class:
We’re hoping to release her to the world as soon as possible – but we have to get the game through the certification process as right now she only exists as a sketch and we need to finish up Borderlands 2! The good news is that the Mechromancer will be compatible with all versions of the game and she will be free if you preorder at participating retailers through the Borderlands 2 Premiere Club!
GameStop's Pre-Order Exclusive:
People will be able to get the Creature Slaughter Dome at a later date no matter where they preordered. It isn't exclusive forever.
We’re entering the phase of development right now where we are focusing on framerate and stability, so it’s too early for me to give any final numbers, but you will see an improvement over the first game.
Level cap plans:
We’re balancing the game to cap at level 50; in the first game we ended up pushing the cap to 69 through post-launch patches and DLC. I don’t have DLC plans for Borderlands 2 yet but we tend to want to one-up ourselves each time, so… we’ll see what happens!