I'm fairly well-acquainted with 5e rules, and realised recently that, as far as I know, being unarmed in melee combat imposes no penalty at all to AC, nor confers any advantage to an armed attacker.
By unarmed in this context, I am referring to the description of a character neither holding nor possessing as a freely-manoeuvrable part of their body something capable of deflecting or making attacks beyond an unarmed strike. Examples would include a longsword, a shield, or an iron bar, but would discount insignificant carried objects such as a stone or a pencil. A general rule might be that if the creature has attack options beyond making an unarmed strike, or is holding a defensive item such as a shield, it would result in the creature being considered armed. As an example a werewolf, a tarasque, and an earth elemental would be considered to be armed in this rule because they all have explicit attack options.
This seems counter-intuitive to me - if character A, armed with a longsword, rushes at layman character B while swinging said sword, who has but his fists to protect him, it seems to me that B is at a significant disadvantage (in terms of avoiding/blocking the attacks) compared to if he were also armed. My first question in this scenario would be: does 5e necessarily assume that the standard adventurer is not capable of using a weapon to parry an incoming strike (and the possession of a Feat such as Defensive Duelist allows this)? If not, how do the rules resolve this with the concept of arbitrary HP loss/damage?
I've been considering this, and have come up with a rule which has me feeling tentatively satisfied: melee-weapon attacks on an unarmed creature have advantage. As a side-note, the Monk's Unarmored Defense feature would allow a creature to ignore this penalty. I like this solution as it functions somewhat-independently of Feat-based defensive options such as Defensive Duelist and Dual Wielder, preventing a disruption in their balancing, while addressing the mechanic that I take issue with. I briefly considered letting a(n unarmed) creature backpedal to avoid a melee attack, but that's a different rabbit hole and can of worms.
This could also come under the category of creatures that are unable to defend themselves, namely creatures affected by the Unconscious, Paralyzed, or other similar conditions, which I would likely house-rule to cause the affected creature to have an effective Dexterity score of 0 (typically -5 to AC), so as to keep these statuses relevant in comparison to an unarmed creature.
How does this rule sit with the overall balance of 5e, and how could it be improved? Should it even be considered?