Argyle King
Legend
str bonus to ac is a terrible idea.
I think plate should give a -2 to hit, even WITH training and the appropriate strength. Try boxing a few rounds in your shorts, then try the same wearing a winter ski suit. 45 pounds, even perfectly distributed around your body, in the midst of life-threatening combat (or even sports combat), would cause you to drain stamina like crazy. Maybe with years of training...but the same guy not wearing armor should hit more often. (but get killed faster too, in trade).
Fighters or those with a certain theme could offset that penalty.
I'd agree that being encumbered would cause more fatigue. I highly disagree that wearing 45 pounds of armor would cause such a penalty to the ability to fight. Granted, I've never worn plate armor. However, I have worn body armor in combat. While the type of armor I've worn is a lot different than that of a knight, I'd like to believe I have some amount of insight.
Beyond that, I don't like the idea of heavily armored D&D characters needing to take a feat to be effective with a piece of equipment they already know how to use. To me, that's starting to edge into 'feat tax' territory. Such a feat or theme becomes too much of a no-brainer choice; even with flatter math.
If we're shooting for more realism and granularity, I would agree that using a shield would give a penalty to hit without proper training. D&D as a system would need to be a lot more detailed overall for that to fit in with the rest of the game though. Otherwise, I feel it still edges into feat tax territory.