The Romans(who to me set the standard on EVERYTHING)protected the body and the head and covered the rest with the shield. Full plate armor during the middle ages evolved into something only useful against other knights or unarmed, untrained peasants. English victories during the 100 year war were mainly because French knights rode into mud and allowed themselves to be massacred. English longbows were actually useless against armor, it was just that the HORSES didn't wear armor.

So, armor is a pain in the butt ALWAYS and you are better off avoiding being hit than trusting on it. Because the things you meet in a D&D world are WAY too much for any armor you can actually fight on.