I like this as a shield. It's a cool idea that has potential to be useful in and against specific decks.
Perhaps increase the cost of the greater shield and give it a 1 Damage Reduction also, this would make up for the fact that it only effects them after they've dealt damage, and also has the effect of keeping enemy creatures at 1 damage cos they can't hit past the shield to trigger the effect.
Creatures that can't attack (0, or 1 on the upped shield) should not be effected by the shields effect. No other shield works that way. Also, realistically, if you did do that, they wouldn't stop at 0. They'd go into the negatives. Making the shield rather OP.