Gothmog said:
True, but then again, weapons also broke like nobody's business in actual combat. D&D having weapons being pretty much indestructible barring a sunder is very historically inaccurate, so I don't see how this feat would really be a problem game-logic wise. And the blow that shattered a shield was MUCH preferable to the same blow that would have finished the warrior without the shield- better to sacrifice the shield than your skull!
Two points (one for this and one for the thread

):
The blow that shattered your shield was pretty likely to cripple your arm as well, often without leaving the shield nearly untouched. Wielding a spear or a bastard sword in melee combat was much better if you wanted to get out unscathed since most open injuries were a deadly threat as well.
For the feat:
Think about something like Shield Specialisation (+1 AC, +2 while fighting defensively, as in the KPG update sheets), if you don't want to screw the existing combat system. Giving a higher bonus to AC would be too much. Fighter builds at level 6 with AC 35 in defense mode (without magic) are already pretty hard to hit, Doubling Expertise's bonus to AC would make it a 40.
IME, shields are fine, the problem with the sword&board style in D&D: You cause less damage, many combats are/should be won by causing damage faster than the others. If someone is difficult to hit, then the opponents go for your buddy the raging barbarian. That's a tactics problem, not a balance problem with weapons and shields.
It's obviously that a team needs team tactics. If someone plays tank, it only makes sense if the opponents attack him and not the lightly armoured spellcaster. If you're the only sword&boarder in a group with two greatsword barbarians and a TWF twink fighter, you gonna get bored.
Check out the available ACs for Fightertypes in D&D with or without shields and compare them to averaged attack values of characters of the same level. Shields make a huge difference here, often nearly halving the chance to be hit by the first attack of someone of your own level.
But you face the same problem as a single polearm wielder: Spring attack is not worth it if your buddies go for full attack action exchanges with the monsters.