My initial playtest encounters with the Knight went well, but I recently pitted the party against a group of enemies that included 4 highly mobile skirmishers (Thri-Kreen scouts) and 2 brutes with a push on their MBA's (Blackscale Lizardfolk). Against enemies like these, the Knight is virtually helpless.
The Lizardfolk weren't a problem for my Knight, per se, because he's a Dwarf. But any non-Dwarf Knight is extremely susceptible to forced movement, as enemies can push you away from their allies (leaving them out of your aura, utterly preventing you from doing your job). With the traditional marking mechanic even if you were unable to punish enemies too far away to violate your mark, at least they still had the -2 penalty to attack against everyone but you.
The Thri-kreen had an encounter power that lets them jump their speed without provoking OAs. Deathjump spiders are another nasty skirmisher that can do this. Point being, the enemy neither attacked an enemy while in the aura nor shifted, so the punishment mechanic doesn't apply. The movement itself doesn't provoke OAs, so they can basically move wherever they want and the Knight can't stop them. The Scout in my playtest got obliterated when multiple Thri-kreen swarmed him, and he spent about half the fight on the ground dying.
This makes the Knight the swingiest defender I've seen in action. They can be extremely effective, but have a situational weakness that renders them inadequate as a defender.