The primary issues for me:
1. Heavier armor requires additional proficiencies. These proficiencies mean little unless those categories of armor are straight up superior--not just for the classes that get those proficiencies, but for anyone.
2. Each and every type of armor (not just category) has to be viable for the entire campaign. Otherwise characters are forced to abandon the armors that fit their style and aesthetic sense in order to keep up statistically. It's the old pre-3e "every warrior uses a longsword or a greatsword" situation.
3. Dexterity bonus cannot over-ride the concerns just listed. For instance, if heavy armor is only better for someone with no Dex bonus, then again, that heavy armor proficiency means absolutely squat, since all it does is allow a fighter to use the only reasonable choice of armor for them, assuming they take Dex as a dump stat. If they don't dump Dex, they will wear a lighter armor, and hence not benefit from their proficiency.
4. Rules need to stay true to the traditional feel of D&D armor and weapons, as much as is possible, though this is subordinate to the above greater issues.
Now, in reverse order, some considerations:
a) There shouldn't be things like DR for standard mundane armors, because no prior edition has had that (adamantine isn't a "standard" armor, and so such exceptions are possibilities). I'm not singling out DR; that's just the one that comes to mind.
b) Armors do not need to remain in categories of light, medium, heavy, or even in categories at all, since that was introduced in 3e and can be abandoned if there is a better route.
c) If having a Dex bonus essentially means that you have equal AC to a higher category, then you actually are better off, not equal, because higher categories penalize mobility. A possibility for dealing with this is to allow full Dex bonus for *all* armors. They only started limiting it in 3e, so it can be jettisoned. The complaints that I anticipate are "why wouldn't everyone take high Dex?" and "what reason would you ever have to wear lighter armor?" Valid complaints on first glance, but it breaks apart when examined. First, you only have limited stats, and in 5e that is much truer than 3e and 4e. Sure, a fighter can max Dex and wear full plate, but then he's going to have to skimp on other things he might need in order to be an AC tank. I think that's a perfectly valid option. Keeping a Strength requirement to wear heavier armor is a possibility to avoid the above situation, and it might also help in other areas.
d) Heavier armors do and should penalize you in ways other than AC. You lose speed and mobility. I think, if needed, those penalties ought to be increased if any balance is needed. Those very considerations of mobility should be what makes wearing lighter armors appealing to some fighters. The standard fighter is going to wear the heaviest armor. A fighter customized for speed and mobility might choose to wear medium armor, or even light armor if he was highly customized, but that would be rare.
e) The only way for point 2 to happen is to make sure all of the other issues are addressed beyond just category. If there is a "best" medium armor, then even if medium armor is overall between light and heavy in effectiveness, everyone will have to upgrade to the best version as soon as possible. Take 3e as an example. I think scale armor is awesome in style. But it's straight up junk armor. It has absolutely no benefit over a breastplate other than being slightly cheaper (only an issue at level 1). The classicly stylish chainmail suffers a similar complete lack of usefulness. I think heavier armors ought to be more expensive, and you might not be able to buy the heaviest armor at first level (just like you can't necessarily afford both an exotic melee weapon and an exotic ranged weapon to start). But by 2nd (or 3rd at the outside edge) you can afford whatever mundane armor you want. This solves the issue of world verisimilitude, since armies and bandits still wear cheaper and less protective (but more maneuverable) armor.
f) Despite that, it's also possible to have one cheap, purely inferior type of armor at the light category for such purposes, just like club is for weapons. g) A possibility is to only have 3 or so types of armor, which are stylistically subdivided as needed. For instance, one line of the armor chart says "Light (leather, studded leather, chain shirt)" providing a set of statistics that applies to all of them. You have a list of which armors are in which category, and can choose the style you like. Then, an advanced module would make distinctions (that keep them on par) for those who prefer to differentiate more. An example might be to make some armors get a +1 vs one type of damage and a -1 vs another. A chain armor might get +1 vs piercing and -1 vs bludgeoning. Mobility adjustments are also possible.
h) An alternate possiblity would be to ditch the traditional categories, and have say, 5 types of armor (each including multiple "styles" as described above.) Different classes can wear armor up to a specified category. Much like 4e, but jettisoning the "light/heavy" layer.
i) Does the fighter wear "the best" armor (and by that, I'm just going to go with "provides the best AC", because obviously we have to decrease mobility in it), or does he just have to pay more to have the same AC and less mobility than the lightly armored rogue? A character in light armor should never exceed the AC of someone in heavy (making them better off, because of mobility), and should only have a chance of equaling it if they have a Dex of *at least* 18. I'd say not even then.
j) We could always ditch armor proficiencies entirely (it was introduced in 3e) and just have class abilities and other things that make wearing different types of armor more viable. For instance, if rogue-ish abilities are penalized in armor above light, guess who won't use it? Same with barbarians and rangers in medium, etc. This honestly will accomplish most of what needs to be done as far as channeling classes into the types of armor they are expected to be in, while still allowing a mobility specialized fighter a valid choice of wearing an armor type other than heavy.
I'm going to here say that I acknowledge these considerations are based on both gamist and narrativist styles. And here's the kicker: I'm actually a simulationist. I'm just not so much of a simulationist that I want characters statistically gimped if they pick an armor that they think looks cool (narrativist), or that should, based on paying for it with class/proficiencies provide better AC (gamist.)
Simply fix armor how weapons were fixed in 3e and 4e.
If I could get one message directly to the developers at WotC it would be this specific posting right here, right now.
