Presumably the monster doesn't want to spend a round hobbling across the board and not attacking when there's a perfectly good monk to squash. Its the decision that most players would make in that circumstance. The incentive is getting to make an attack instead of not making an attack.
Whatever, I'm not going to argue it. I think its pretty clear that powers that force an enemy to hold still, adjacent to a melee character, and attack that melee character are Defender powers. If they aren't, there's no such thing as a Defender. I think this is clear enough that I do not need to defend it.
Actually, in a way a Controller is a Defender and vice versa, but as you see, the most important difference is that the Defender makes himself a target, the Controller tries to make no one a target.
Immobilize or Slow an enemy at range means making hard for him to attack anyone (if he's not using ranged attacks). Do the same in melee, and you're his next target.
There are also things defenders are worse at then controllers - targeting multiple enemies at range, for example. The Fighter has a few abilities that give him limited range (Come and Get It!), but it ends up with him being adjacent to anyone he hit. Again, making himself a target.
Defender and Controller similarity might be: It's a good idea to engage them. The difference is: It's easy to stay engaged with the Defender, but hard to engage the Controller. The Defender can survive being engaged, the Controller cannot.
But this is very simple, and mostly applies to Fighter/Wizard as our archetypes for defender and controller. A Swordmage already acts differently - he will often try to get away from the opponent, and doesn't really want to be engaged.
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A Striker that deals massive damage to have an opponent stop attacking a Controller might be "defending" him, but that doesn't mean a Striker is a Defender. He has not the hit points to keep "drawing aggro". That's exactly what happened if you let a Monk Full Attack in 3E - suddenly he becomes annyoing (well, if he manages to hit with his
Flury of Misses), and his enemy might just drop him with his own full attack - or at least force him to run away. He can buy you some time, but he cannot sustain this position.