Classes gaining the ability to cast magic-user spells, such as the ranger;Personally, I don't care for the hand-wave of "enchanted armor is ok". I believe, and referee this way, that it is the encumbrance of the armor that is the culprit. There are movements required in spell casting that can be rather nuanced.
Elves, multi-classed, and dual classed characters.
Fighters are practically born in armor. It isn't simply a matter of becoming accustomed to the weight. It is also a matter of learning to move, to perform mundane tasks, such as feeding oneself. Armor is surprising articulated, but it is still a heap of metal obstructing and interfering with joint function and range of movement. Fighters devote a lifetime to learning the subtleties of operating in armor.
Anyone not committing that level of dedication can't hope to do more than walk across a room. The intricacies of spell casting are far beyond their capacity while armored. Furthermore, even if eschewing spell casting, and attempting to don the armor to survive a pending attack, they will be woefully unable to take any advantage of the armor. Absorbing, deflecting, and blunting blows have not been their priorities, and simply encasing oneself in steel isn't enough to call oneself "armored".
So, for my money, no one can cast magic spells while armored. Even if their class allows "Any Armor and Shield". My only possible exception to this would be to allow Rangers to cast while wearing Leather armor. Maybe. If I was feeling really generous.
So, how about it? How do you guys deal with this?