These are some important things, what ares has mentioned here. It could make shieldfighting more active than just holding the right mouse button and letting the enemy hammer at your "magical force field" until it breaks, before he actually becomes a threat to you. I had a few situations like that too, when you just ask yourself why your damn weapon can't hit the enemy, even if the shield of your enemy was miles away from where you hit him. So please, do some work to shields, it will not only make shield fights more "realistic", but also more "active" and thus: more fun!
About the crouching: I think there has to be something to prevent this being abused like bunny hopping (as was told before by someone), but all in all i think crouching could really add something to the gameplay experience.