He used to be able to get right up behind the striker and basically boot them in the back, but at the same time not manage to draw a foul against him. No he's just bowling players over and acting a bit thick. You're right about his decision making going to pot.I think he was always a bit erratic but under Mancini there was much more focus on being defensively solid. Pellegrini has opened the team up and over time Kompany has started to become more exposed and take more rash decisions.
He's a defender who is too eager to win the ball, that's his biggest problem.