Traditionally, the lowly shield garners a basic bonus to Armour Class, as needn't be explained here. The problem many outcry is simply how little an impact one of the most useful tools of war has, in foot melee in the realm of Fantasy Role-play.
The shield is cried to uselessness, after all, it's guaranteed the higher-level monsters will beat non-modern AC practically every round of combat. Some are fine with the small bonus of a shield, given that getting hit is so inevitable, but that the bonus is still useful against the more common and mundane foes expected to combat in the original game.
How can we fix or circumnavigate this issue? Simple; a new mechanic, and one that is very simple. But is it really necessary, especially when combat is supposed to remain abstracted by game design? What would a chance of blocking individual blows mean for this concept? especially when it would apply equally to the already disputed nature of arrows and bolts in the abstract combat (should we exacerbate the problem further?).
For some time now, how I've run shields is a bit different from the norm, so I shall make them known. Shields provide a bonus of +1, as the norm, though wooden, hide, and wicker faces do not benefit plate armour, instead requiring a metal shield (preferably kite). Without other armour worn, shields count as AC7, and tower shields as AC6 (normally complementing armour by +2 AC). Archers' shields and other small types counted as normal shields, except without bonus against missiles. Shields could be worn across the back for their bonuses against rear attacks.
This original effort varied their place somewhat, but it still was mostly aesthetic. There wasn't a strong effort to make shields too good in the game, considering the other options available to Fighting Men; two-handed weapons rolled two dice, taking the better with +1 in Hits; and using a second one-handed weapon to deke for a +1 To Hit. So I couldn't lean too much favour. This changed somewhat with the design of offensive shields (tweaked from the tail sweeping mechanic I gave dragons).Shield-barers (excluding bucklers) could opt out their shield bonus to bash with the shield, and any time a '6' turned up, the roughly-man-sized and smaller foe was flattened and stunned for one round. Tower shields did so 1/3 of the time.
Now, this finally brings me to my point and suggestion for shields. Keeping them with their +1 bonus (or dropping it), any hit against a shield-barer is reduced to minimum damage (1 if 1-6, 2 if 2-7, etc.) 1/3 of the time (rolling a 2 in 6 chance). Tower shields could do so 1/2 the time. It's simple, elegant, and best of all, continues finding more uses for those sexy six-siders. The reason it doesn't outright negate an attack's damage is to reflect the savagery the guy behind the shield is feeling regardless. A missed roll to reduce could be seen as not bracing his shield arm well and getting battered around, even if the shield is being hit.
I'd like to thank that Paladin in Citadel for inspiring this post.
For those interested in that dragon attack; the tail swipe was randomly determined (and depended on how the party was fighting it), and dealt one die of damage per age category (!), chopping all in a certain radius (distance in inch spaces by category). Every time a '6' turned up for each person hit, was how many rounds he was stunned and flat. I wanted very dangerous dragons, and the older ones could batter down wooden walls and swaths of forest with ease, so be wary.