The best zone defenders will play toward the rear of the zone and try to keep everything infront of them. This way any crossing routes can be easily picked up, as can dump offs and QB scrambles. Most players also face the QB and play at an angle with there back toward the sideline. In Madden, they really don't portray that well.
When you have a player in your zone realistically you want to force him toward the center of the field, where all of your help is. That is where Madden really fails, defenders work as individuals in areas, making hand gestures as if they are passing off but not going to the boundary of the zone to do so. This really exaggerates the gaps built into zone defenses.
Even worse and perhaps most damaging is the fact when a defender has no player in his zone, even if no threat is in the area he will just stand still.
On rare occasion I have see a defender in the flat appear like they are going to blitz if nobody is in the flat, but it's few and far between.
Years into the game and this still frustrates me to no end, as a new player I can only imagine the confusion and anger that could occur.