Addictive game play and loot, funny dialogue, and great co-op play with tons of replay value.
The game doesn't have the greatest graphics, but the world has been greatly improved over the original; there are various locales that have their own feel, and many more baddies to fight. You're not just killing bandits all the time.
The basic shooting mechanics are very solid and well thought out - using the right elemental weapons makes things much easier. It's a rock paper scissor approach that plays well. Really, early on you want to make space in your backpack for the different weapons - and that little hint of strategy and planning makes shooting far more fun. And looting as well.
Another cool feature is the badass system; as you amass badass points, all of your characters can use them. It doesn't make the game too easy, but it helps early on for new characters.
There's also the True Vault Hunter mode, which is new game mode. It's fun and challenging. I'm in the middle of TVM, and the loot has been a bit disappointing, but that's just random luck. I got a legendary at lvl 8 on one character. The game just makes you want to keep killing stuff in that never ending questing for the ultimate loot (although I refuse to grind bosses).
Online co-op is easy and fun, it's just running and gunning.
About the dlc: the mechromancer is a *must*, she is very fun to play and has a lot of great kill lines. I would say the pirate dlc is optional, and I was going to wait for it to be cheaper but I was playing the game so much I decided to just buy the season pass. Buy the game first and see how much you like before you invest. But you definitely should get the mech.
Bottom line: a fun, addictive game that puts a smile on your face. What's not to like?