So I caved and bought it, here's my thoughts:
The improvements remind me of the year that Kobe was on the cover (when they stopped letting you just huck the ball from side to side) in that the 'fluidity' improvements might sound subtle, but they're actually pretty big upgrades to gameplay.
Not getting 'stuck' after a physical contact make guys like Josh Smith and Dwight Howard more effective. This year on a break if J-Smoothe gets the ball in transition and a smaller guys in the way, he more or less powers his way to the basket. In year's past, you'd get that awkward animation when he gets hit further from the basket and obviously doesn't hit the shot. Same with Howard: His post moves, realistically, aren't all that good - but you can definitely use his size and speed on the block to get him easy hoops.
Another positive I noticed is that 3's seem to be a lot less easy. I couldn't do that old trick where you just push the ball up with the pg and hit a wing at the elbow. Even good three point shooters need to be closer to the line, and generally more in rhythm. Feels like they made a conscious effort to have a more balanced approach to offense.
There are a few things I hate: It might be one of those things where it just takes time, but I freaking hate the new post system controls. In my first game it legit took me like 15 attempts to get either Drummond or Monroe to take a shot attempt. And even after I did, it wasn't very pretty. I liked 2k13's post system a lot: Thought it was easy enough, while allowing creativity. So far, this one just stinks.
Also, they changed the pick and roll button to the 'b' button on 360 (i'm sure it'll be circle for ps3) which might seem minor, but to me it's super obnoxious. Holding the LB and operating pick system always felt fluid, this is really awkward (and I don't htink just because it's 'new', more to do with what your fingers have to do).