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This is really minor, but here's my take.

Disarm
If the creature is armed with a melee weapon and you arn't, you have disadvantage on your check.

I don't know, but when swordfighting irl, I find it actually easier to disarm someone with a free hand, especially a sword, all you do is grab the pommel and turn the right way - it doesn't even require strength, you just have to know what direction to twist (the pinkie points the way!).

I think the only thing that should cause disadvantage is if the opponent has a two-handed weapon... that's always difficult.

Also, I think combat skill is important, in 3E, you had to make an opposed attack roll, to me that makes more sense. Anyway, its one of those things that's really is to House Rule.

I believe the reason for disadvantage is that they have a dangerous weapon and you have soft easily damaged by said weapon, hands. Though I see that being slightly less of an issue with clubs or maces or hammers than with swords and axes.
So, the rule makes sense to me. 

It also leaves open an option for monks (or the right kind of fighter) to ignore such a penalty.

I think the idea is that you catch or deflect an opponents weapon with yours, and then disarm with a free hand or a fancy strike to the opponent's hand. 

Usually if you don't have a weapon its because a) you are surprised, b) you have been captured, or c) you were disarmed last turn

All of these situations seem like a good idea to penalize your attempt to disarm someone.

I guess I would house rule that if you readied the disarm action until after the enemy hit or missed you, then you could disarm without disadvantage.