I completely agree with you, I can't even begin to count the hours I put into 2k11 I played it almost every day from Midnight of it's release day till the demo dropped. Now I've been playing the demo every day. The differences are profound but they don't jump out at you because ironically they're changes to the fundamental non-flashy aspects of the game.
The difference in movement is a huge difference, the rebounding, the way the ball reacts, changes to the defensive stance mechanics. Things that probably wouldn't jump out at someone who didn't intimately get to know 2k11.
If the retail makes the demo feel sluggish than I will be one happy gamer come Oct 4.