I won't be too specific because I'm no game designer, you guys are better at specifics than I could be.
I feel that it is vital that you don't fall into this "everything equal" trap. For example, robes would give you some magical shield bonus, leather would give you some dodge bonus and plate would give you damage reduction and overall it'd be balanced to all offer the same protection, please don't do that, one of the benefits of playing a group of characters is that they can have weaknesses that they cannot overcome by themselves.
I can't wait to find out how you want to make every kind of armor viable for classes in a way that different class specs would desire different pieces of armor, but the basic archetype should be clear:
Plate offers the best survivability period. A heavy fighter covered in shining plate from head to toe should feel heavy and durable.
Leather is all about quickness and stealth; leather is fragile, a thief in leather should not walk in the open and it should feel dangerous to do so.
Robes confer power and offer no protection whatsoever. Wizards are above the rabble that needs to wear unwieldy protection to fight.
The important part is that this concept does not change depending on the stage of the game, so many games make everyone powerhouses at some point, robe users should always need special protection, a heavy fighter should always feel awkward to fight with without backup, and someone in light armor should always be alarmed if an arrow flies his way.
Concerning the stats, I'm actually fine with a somewhat simplistic system, the next tier all things equal should be better than the previous tier and there's nothing wrong with having some generic modifiers (I don't care how that is communicated). I don't know how much depth the system should have, too much depth would probably detract from the experience rather than add to it, to me it is more important how the armor is aquired and how its value is communicated. A +3 item in Baldur's gate, while plain, was more exciting than a shiny yellow rare item in Diablo for instance and that felt good.