Here's what I'd recommend if you go with a power. Note this is a rough design but hopefully something folks could use as a springboard for something more polished:
Disarm
At-Will * Weapon
Standard Action Melee Weapon
Target: One creature
Special: This power triggers an OA.
Attack: Strength - 2 vs. Fort OR Dexterity - 2 vs. Reflex
Hit: The target drops it's weapon at it's feet.
Notes:
(a) It takes a minor action to pick up a weapon.
(b) You suffer a -2 penalty to the attack roll if the weapon is being wielded in 2 hands.
(c) you suffer a -2 penalty per size category the target is larger than you.
And much like the cheese that comes out of WotC due to the laws of unintended consequences this becomes a very high chance of defeating any weapon using solo every single round at even low heroic levels since there are plenty of ways to ensure that your hit chance is through the roof.
Party members delay and attacks right before other party memebers, not giving a fig for the special; if he hits the opponent will be attacking him unarmed and even the occasional OA is going to be a lot less damage than having the weapon user attack the party for the entire combat.
Then other party members push, pull, slide or teleport the opponent away from the weapon or the weapon away from the opponent and your carefully crafted boss fight is pretty much over. Unless of course your big bad guy has a couple of dozen swords, but the players would probably get annoyed with that pretty quick.
You want a drawback appropriate for this level of instant win?
Special: Under no circumstances can you reroll this attack and if you fail to hit your weapon flies 10 squares in a random direction and you take damage equal to a healing surge; if you are attacking unarmed you take damage equal to your bloodied value.
There are plenty of reasons to not allow disarming at all in 4e:
From a simulationist perspective, it's hard to disarm a capable combatant in real combat. It's easier just to kill them and take the weapon from the corpse. In 4e this ties perfectly into the 0hp = dead, captured or disarmed mechanics.
From a gamist perspective, the hero of the piece fights the good fight, the duel raging back and forth across the deck of the burning ship for a couple of minutes before finally forcing his opponent back against the rail where he disarms him. You don't see the hero swing over on a rope and disarm the foe he's been planning vengence on for a decade a mere two seconds into the fight. This also ties perfectly into the 0hp = dead, captured or disarmed mechanics, the only foes weak enough to casually disarm are minions.
