After watching the Spartan vs Ninja episode I was surprised at not only how much protection the Spartan's shield provided them, but also how effective it was as a weapon. Blows from the edge of the heavy shield were as deadly as those from other weapons. This definitely supported my ideas about making shields more effective in D&D by blocking damage - but made me think they should provide some offensive benefit as well.
Building on previous house-rules, a Fighter with a shield benefits from the rules in the earlier "In Defence of the Shield" article:
A character with a shield may roll a single d4 for one attack causing damage on them per round. If the roll is greater than the amount of damage from the attack, the shield blocks it and no hit points are lost. If the roll is equal or less than the damage, the attack causes the regular amount of hit point loss.
In addition to this, a Fighter using a shield is also considered to be fighting with two weapons (from the "Choose Your Weapon" article):
Characters fighting with two weapons roll to attack normally. After rolling for damage with their primary weapon, they may choose to re-roll using the dice for their secondary weapon. This second roll is used for damage, even if it is less than the first roll.
When used as a weapon a shield does the same damage as a club:
Shield - d4
A shield may not be used as a weapon in the same round that a defensive shield roll is "given" to a nearby ally.
I think it makes sense to restrict this use of a shield to the Fighter class. Other classes can still use shields defensively, but only the Fighter's martial training should allow them to benefit from using a shield offensively. I could even see it being something that's only introduced at the 2nd or 3rd level, or that requires a minimum strength score. For now though, I'll plan on letting all 1st level Fighters use their shields as weapons.