It's not just the coaching part where Houllier falls down compared to Benitez (not another Houllier dig by the way, I have the upmost respect for a very likeable, intelligent guy), it's the tactics as well.
The zonal defending system is one where being a unit really is the key, when the opposition has the ball, you must all move as one and read the game, when the ball goes backwards or sideways, they all move up to try and catch any attackers on the back foot, it's quite a simple theory really, but quite hard to put into effect. Benitez and his coaches are masterful with it though, as has been shown.
With Houllier it was just a case of defending deep and making other teams play in front of you, defenders hate having to face their own goals. Then we just smashed teams with our pace on the counter. Very effective with the right players, but easily nullified by good teams and good systems.
As I say, not another anti-Houllier post, but a pro-Benitez one.