People are right to consider the controller to be close cousin to the defender - both roles are focused on enemy action.
To go through all the roles:
A Striker is all about applying damage - both in large amounts, and where desired. Strikers have bonus damage with mobility/ranged (sometimes both of these). Focus fire is easy with strikers.
The Defender, on the other hand, is basically the anti-striker. His job is to make sure the monsters can't strike effectively by interfering with their ability to target the characters they'd really like to get to, or reduce their damage by making their most readily available target tougher and harder to hurt.
Strikers apply concentrated damage to kill desired enemies, defenders keep concentrated damage away from desired PCs.
The Leader makes the whole of the party greater than the sum of its parts. He keeps his teammates alive when they take lots of damage, helps to remove conditions that keep them from acting effectively, help get them where they need to be, or provide bonuses so their actions are much more effective. IMO, one of the keys to playing a great leader is knowing how your allies work so you can set them up and support them properly. When there's a monster that needs to go down, it might the strikers of the group doing most of the damage, but the leader is probably helping to make it possible in some way.
Now the Controller is basically the anti-leader. Where the leader offers powers that improve the plan of his allies, the controller takes apart the plan of the enemy.
Just like the Leader gains a lot from knowing about what his allies will do, when they'll need heals, what powers help them the most, etc, the Controller benefits from understanding what his enemies are going to do. Then he messes them up. Got enemies coming down a flank without any of your melee friends to cover? Icy Rays pins them down for a bit. Is that Artillery unit going to keep shifting away from your beatdown squad - you can use Icy Rays here too so it has to provoke. Melee enemies usually need to cluster to focus fire - and area powers can mess that up.
Of course, just because the controller is concerned with enemy action doesn't mean coordination with his friends is unimportant. Friendly fire sucks. If your allies aren't careful with positioning and shifting, then a lot of area powers will either hurt your friends (which is bad, especially when you impose conditions) or hit very few enemies. Similarly, having a lot of Slow, forced move, or Immobilize powers might not be useful if you have a strong melee team. Our invoker often found herself not moving enemies, since we already had them flanked (which our rogue often needed).