LostSoul
Adventurer
Here's the #1 reason why Disarm doesn't work in 4E.
Open the MM and look at a random monster. Here, I'll do it -- Earth Titan. It has a +20 to hit with its basic attack, and it does 2d10+6 damage. It's described as using a greatclub, so presumably that's what it uses to attack.
Now, knock the club out of its hands. What does its attack look like now?
It smashes you with its fists of stone.
Not as kick-ass as the greatclub. Let's turn to page 42 and see what we get. Level 16 Brute - let's use the medium column. 2d8+7.
This could be a fun game. Let's pick another monster at random.
Drow Blademaster, page 94. Sounds like disarming this guy will be a good tactic.
We disarm his longsword. He's already doing low damage, so let's lower it even more. 1d4+5. Important - he can't use his fancy longsword powers now either.
You can argue that disarm is too good against the drow blademaster. Sure, I see your point. But maybe it should be that good against this guy.
At any rate, I'd probably only allow disarm 1/encounter.