armors
If platemail and medium armors are properly balanced, i.e. they ALWAYS give equal or better protection, no matter your dex, vs their disadvantages in the wilderness, that is a win-win.
I doubt a ranger - or anyone - would survive for very long periods in the wild wearing platemail all the time. Sure, if you're on horseback, or going off to fight a major battle, put it on, but it shouldn't really require training IMO. Wizards could have their arcane failure chance to hold them back, but I can't see why Aragorn or Conan shouldn't wear platemail into battle if the need arises.
What I'm saying is...the balance of platemail vs not platemail should be one of : it's heavy, slow, cumbersome, makes enemies aware of you a mile away (especially natural ones), and there is just no way a self-respecting ranger would bother using it, outside a big battle to save the kingdom.
so yes, I concur, being lightly armored is the archetype, but it should follow naturally from a set of non-combat rules that would make it very, very hard to survive for long periods in the wilderness wearing a suit without a horse, an entourage, squires, oils, slave girl massages/rubdowns, whatever floats your boat. Also, the cost...if platemail's cost were properly balanced vs the expected gold income, even of a semi-successful ranger, it would be a luxury that most wouldn't bother with, considering the negatives of owning it.
E.g. I own a real sword, but I can't take it outside and swing it around or I'll get arrested. That's a practical reason not to wield it. Plate mail should be uber tough, INSANELY expensive (forget about it before level 5, in most campaigns with standard gp values), and even then should require upkeep and give you such negatives to your survival out of combat that you'd be more likely to keep just the breastplate on than the whole thing.
Maybe make Plate armor give you negatives to hit (clumsy / slow + impedes your vision), until you get high enough level and specialize in it...that'd be one example of a mechanical disadvantage that would make players think twice even if cost were no object.