Far less involved and far more likely to be applied during play than my rules for armored mages are my house rules regarding the thief class and armor. They go as follows:
Thieves may opt to wear armor other than leather, but doing so limits their effectiveness in their chosen profession. Wearing armor providing up to AC 6 (scale mail) will reduce a thief's abilities to 1/2 normal (i.e. hide in shadows, move silently, etc...). Wearing armor providing protection of AC 4 or 5 reduces the same to 1/4 normal. A thief may not use his special abilities when wearing armor providing protection of AC 3 or greater.
Some common sense still needs to be applied. A thief capable of reading languages should be able to do so regardless of how well-armored they are. A thief climbing walls may need to put down their shield to even make an attempt.
My purpose in offering this rule and the one for armored mages is not to have iron-clad footpads and wizards traipsing across my world. Rather, I want the players to have options. Even bad options are better than a flat "no" when asking, "Can my character pick up and use that shield?"