I have noticed this from watching people's demo vids so far - player momentum is not part of the shot blocking control...the players only jump straight up - their direction and momentum do not cause the block animation to be a "moving jump" or a "momentum jump" when they attempt a shot block. If you are running full speed at the hoop and you press the shot block button, your momenbum will NOT carry you toward the hoop as it should...instead the player will stop and just jump STRAIGHT UP.
EA needs to add player momentum to the shot blocking the same way they added it to shooting - for example, pressing forward and the block button causes the player to jump forward and attempt a block. press right and block button causes the defender to jump to the right as he attempts the block, etc.
The left stick sensitivity should also allow you to control the distance and power of your jump on the shot block - if you are running full speed to the right and hit the button, your momentum should cause you to do a "long, one legged jump" to the right as you go for the block. if you are barely moving the left stick to the right, your player will attempt a "shorter jump" as he attempts the block.