Controllers are confusing because even WOTC doesn't know what they do. It was stated in that post a while back from someone at WOTC that they originally thought that controllers were about AoE attacks. But then they sat down and thought "Do we want to prevent other classes from attacking more than one target simply because they aren't controllers?"
So, they decided to redefine what a controller was between PHB1 and PHB2. The post said that their new definition is that they are the opposite of Leaders.
Which is still a broad, poorly defined role. Are they the opposite of Leaders because they deal damage to the enemies instead of healing their allies? Nope, everyone does damage to the enemies. Is it because they give negative status modifiers to the enemy instead of positive modifiers to their allies? Unlikely, as nearly every class gives negative modifiers to the enemies so that isn't unique to controllers.
The only thing I can think of is that controllers are now supposed to be the class that prevents the enemy from working together. Which is the opposite of leaders who help the party to work together.
The idea being that using forced movement, negative modifiers, AoE damage, and other status effects you can prevent the enemy from being close enough or have enough actions to aid each other.
Unfortunately for controllers, before WOTC came up with this definition for controllers, they had already pretty much given their shtick away to the other classes. And they had published one controller who was much better at just doing AoE damage than they were at "controlling". Even with the new controllers I can see a lack of focus. They still don't have a solidly defined role.
