Well, the idea of a shield is to make you untouchable. If anyone has the chance to join a LARP, try to form a line of shields and approach a group of others to attack them. Even if they are double in numbers, they won't know what hit them, because you are just not going to be able to get through that freaking wall in front of them.
On the other hand, a shield is a huge wall not only in front of those who want to hit you, but also in front of yourself, so you are not going to hit everyone as easy as if you were able to move around without that thing slowing you down and blocking your movement.
So you could implement a stunt like this:
Sword and Shield:
You have learned to use your shield to protect yourself from an attack against you which gives you a +1 to any defense roll. If you took advantage of your shield like that in an exchange, any attack you try to do is made at -1. If you made an attack before you had to make a defense roll and did not apply the -1 penalty, you are not able to bring up the full defense of your shield this exchange, thus do not gain the +1 bonus.
This is a little too crunchy for me and not nearly powerful enough.
Using a shield does not make attacks weaker. It makes them stronger because you can know exactly where an opponent's weapon is when they try to strike you, allowing you to commit more to attacks.
To me, the disadvantages to using a shield are:
1. They stand out in the modern world. 'Nuff said.
2. Depending on the material, they can be broken.
3. Most are large. In fact, I'd give a buckler a +1 to defend, a heater shield a +2 to defend, and a tower shield or kite shield a +3 to defend. The kite and tower shield would not make supplemental moves possible anymore, though.
4. They stand out and make the bearer a target.
It makes sense to me to make shields that powerful. I mean, even a ghoul would have trouble getting around a shield wielded by an untrained vanilla mortal. Sure, they could grab it, but they'd probably be getting cut up a lot in the process.
I think it makes sense to add the shield to a block rather than armor too because.
A. Blocking with a shield takes skill and is an active sort of thing - not passive.
B. Attacks that get past the shield can still hit armor and do no damage.