OK, so here's an idea...
Swipe the feat and skills system from D&D and make it so that every class has certain class defining abilities, combat bonuses, and skills, but leave it up to the player to decide best how to burn those limited upgrade choices. In that way, for example, you could have a player with a mage choose to be "proficient" in wearing plate armor, but in doing so sacrifice the next upgrade to a class-defining spell.
Here's the thing: Scouts cannot sneak in plate armor, rendering sneak attacks usesless; the armor just makes too much noise for enemies to be unaware of their presence, no matter how sneaky they try to be. Metal armor restricts the ability of a caster to gather the forces of magic, causing there to be a chance his spells will fizzle / fail OUTRIGHT or worse yet, BACKFIRE, and will also incurr a penalty to focus because the mage has to concentrate harder while fighting the limitations of the armor to even make a spell work properly. Druids wearing plate could not shapechange and use their speed buffs, and their own divine attacks would be limited as their dieties would frown on them for using such an "unnatural" armor. Monks and bruisers wouldn't get the benefit of their natural avoidance, as they are unaccustomed to fighting in heavy metal armor. Indeed, they would also incurr penalties to damage because the armor would slow down their normally fluid movements.
The problem is that there are architypes in every fantasy game. These all suppose a certain set of class skills, knoweldge, and abilities are learned by the character prior to his adventuring life - say at an academy, college, or from the military or local mage tower, monastary, or whatnot. This is why a guardian would start out as a guardian - he had some prior martial training and learned how to use swords, shields, and all armors effectively. He even learned how to enrage an opponent and keep his attention (hopefully) off his comrades.
Taking the concept of the architypes into view, it is obvious why all classes do not wear plate and are not meant to. A mage, as he studied, did not have the time to learn how to fight wearing armor and wielding sword and shield. His studies in the arcane were so strenuous the most he could be educated on was how to protect himself with the simple tools he already used for his class - daggers, wands, rods, and staves. He was also taught to fight at a distance, to let the warriors close in for melee when at all possible.
A monk or bruiser were taught the mysteries of mystical inner forces, and how to use those powers to toughen their bodies, hit as if their fists were metal weapons, and they were conditioned to fight in light armor to make use of their lightning reflexes and hurricane punches and kicks. This training is so rigorous and intense, they had no time to learn the intricacies of tactics when fighting in plate mail or chain. In fact, they were encouraged to let their inner strength give them a tougher hide, to avoid and roll with the hits as often as possible - flow like water, sting like bee. Druids worship nature and are taught to abhore metal armor (and in truth weapons, but, eh..) and so they avoid it as it is unnatural, manmade, and corrupted. The druid abhorance of metal armor is their choice based on religion.
Clerics have a place on the field of battle, so they must learn to use martial armors and weapons just as the plate-toting warriors must. They have to be able to be field medics, and have to be there side-by-side when the warriors go down or take lethal blows. Their dieties encourage this strength in protection and warfare, and so they are encouraged to bare the armor of their kin, to be courageous as fighters and to honor them on the field of battle.
Scouts are trained to scout, and must be stealthy and agile for purposes of reconnaisence (sp?) and the somewhat sneaky missions they are required to undertake. They are trained up to the use of chain, and how to move fluidly and stealthily in said armors, how to strike from the shadows and unseen. They could do this in lighter armors to be sure, but their training is so that they could survive the field of battle insomuch as they could return with their all-important information, or avoid capture and death at the hands of the law and the just. Simply put, they could not accomplish the stealth required for their missions if they were to bear clunky and heavy plate armors.
I would guess that these themes would be some of the developers' premisses regarding the class architypes. Sure, there are examples in fantasy of other possibilities, as stated by another poster. But in the fantasy world of EQ2, this is how it works. It would be nice to see a system in which a player could customize his class in these areas somehow - like a berserker being able to truely fight like a roman spartan or a grecian berserker, or even like a barbaric wildman of the ancient germanic tribes. Sure, there would be trade-offs to this within the system, somehow, as there always is. Typically this becomes the sacrifice of dps or abilities in favor of other strengths and weaknesses. I am not so sure EQ2 could make this customizable a system work with the current architypes.
For the most part, EQ2 has crossed some architype lines through the AA system. For example, rogues being able to tank sacrifices some dps or other useful abilities in their AA trees. Sorcerers and enchanters often have to choose between dps, utility, and resistability in order to define their characters' niche. Summoners have to choose what their favored pets are and whether to boost dps or utility spells, making them unique amongst others of their classes. Guardians can be towers of physical protection and hate gathering, or dps wild fighters. Some of the AA trees and the options available aren't that favorable because the class architype might be a difficult one to justify certain options. Many options are situational, and thus create a 'specialist' area that no one really wants because they'd rather play the class another direction.
Regardles... should all classes be able to wear plate? No. It's not necessarily unbalancing. It just plain and simple doesn't make sense for the class architypes in the EQ2 setting.