I have played both, and here are some of the biggest differences I've noticed:
- You can shift into post mode with one button press
- Ally oop passes are easier
- Pump fakes is a lot harder to defend this time, you can easily get someone into the air and draw a foul.
- Improved hot/cold steaks and 3 point shooting. Its a bit easier to make those open shots now, especially on a hot streak.
- Passing is still a bit iffy.
Player Mode changes:
- You now earn a salary. You can use that salary to purchase additional skill points, increase your skill cap, and other things (such as improving team rating), buying dunk packages, etc.
- When your rookie contract expires (takes many seasons!) you can start negotiating with other teams for a better contract.
- There are more key games, and you can simulate to play the next key game from the main menu rather than having to go into the game and choose simulate until end of game.
- The previous height limit for positions have been removed, so you can make a 6 foot center if you wanted to :P
Because of the lockout, the entire rookie class are randomly generated names (really sucks), rather than the actual rookie class of 11.
I haven't explored the other modes in depth yet, but this feels more like a complete game than 2k11 was, at least to me.