Falling Icicle
Adventurer
"Wizards cannot cast in armor" is completely metagame and explaining that armor disrupts the gestures is a poor handwave (the rogue has no problem with dodging and opening locks in his armor, after all).
But what if it's contact with metal that disrupts spells (and spellcasters not only avoid metal armor, but also any kind of metal weapon or jewelery, and putting them in chains cuts off their magic)? This makes it completely understandable why most mages only wear robes or leather - and a rare, powerful one, uses dragon scales.
The idea that metal interferes with magic is just as metagame and a poor handwave as the idea that it interferes with gestures. The 2e PHB actually addressed this idea. "There are even unfounded theories that claim the materials in most armors disrupt the delicate fabric of a spell as it gathers energy; the two cannot exist side by side in harmony. While this idea is popular with the common people, true wizards know this is simply not true. If it were, how would they ever be able to cast spells requiring iron braziers or metal bowls?" 2e PHB, p. 42
Or, one may use the Scarred Lands idea that magic generates a lot of heat as a side effect, so any kind of clothing that's heavy and hard to take off is a risk of dangerous overheat or burns. Wizards wear robes because it lets them cool easily.
This one makes the most sense, at least until you try to envision a master of ice and cold spells overheating from using too many of her spells.
"The 4e approach, where proficiency in good armor is hard to get, but the armor itself does not interfere with spellcasting is also good. It's intuitive and does not leave any strange restrictions that must be explained somehow.
I agree that armor shouldn't interfere with spellcasting, but I think that armor proficiencies should go. Instead, armor should have a minimum Strength requirement, and you suffer penalties for wearing it if your Strength is lower than that number. The idea that armor requires as much training to use as a weapon is a ridiculous. The hardest part about using armor is putting it on (at least in the case of things like plate mail), and knights had squires to help them do that.