Originally Posted by Gnorman
Another possibility for reining in mage classes: d4 HD and no armor proficiency.
The Mage class chassis doesn't really need any reigning in, as any problem stem from the actual spells and abilities; artificially weakening mages to somehow "compensate" only makes them squishier in a game where healing is much rarer than in standard d&d and where resurrection is almost unheard of. The fact that most casters can't have access to all the buffs usually used to compensate for their little health and armor (things like Mage Armor, Blur, Invisibility, and all the other spells that make arcane casters somewhat durable) already accentuates this.
Also, the d4 is just too small for a player character, especially in the post 6th level experience; it's why Pathfinder removed it.