One note on combat challenge at reach.
Polearm fighter stands behind character x [using the party I DM for, the highly intelligent, heavy shield and hide wearing warlord whose AC is much better than the fighter, even without factoring in he mark]. In front of the Warlord is the marked foe.
Now the foe can:
move to get to the fighter and attack him, provoking from the warlord
make a ranged attack against the fighter, provoking from the warlord
he can attack the warlord, invoking the fighter's challenge
he can shift to get get to the fighter, invoking the fighter's challenge
So unless he has a close blast power that can target the fighter [who has cover thanks to the warlord] he doesn't have many choices for attacks that don't result in him getting hit. The paladin's divine challenge was changed to avoid situations where you mark a guy and he has to take damage [or in this case an attack] no matter who he tries to attack.
From our games, the "mark the guy and use ally as cover" trick is a trade off, you lose the interupt option, but you get to give both characters +2 to their defences, in a way.
You can do more OAs per round, but the conditions for combat challenge are much easier to pull off, and thus will come up more often [or restrict the opponent's actions further].