He's waiting for the ball to come to him and he's not attacking space to receive a pass. That's why most of the time he finds himself with a defender to beat and being predictable (you just know that he'll try to cross with his right foot), he's easily neutralized. With Pirlo in midfield it's really stupid Krasic not to run in space without the ball.
That's his biggest weakness though. Tactically he's one of our most limited players and he has the worst off the ball movement in our team. What a shame. With his qualities he'd have been a hell of a winger if his off the ball movement was better.