The thorn carapace that affects players is only really useful in poison decks.
A deck doesn't have to be built around poison for poison to be useful, you know. It's still damage dealt. Say your opponent has a Lobotomizer out. In five turns, the poison shield will match that weapon's damage in poison. After that, it's outpacing the weapon. Many, many decks use weapons as part of their strategy, and I can think of lots of reasons a cheap disincentive would be a great thing to have.
The half-damage shield concept isn't really OP unless you're talking about huge creatures, for a few reasons:
1) Weak creatures are usually cheap, and will probably be dead soon anyway. (Exception: Armagio, but it's not for hurting things, and 1 can't be halved.)
2) Huge creatures cost a lot, so they won't come out very quickly. You'd either have to wait and Plague them all, taking a lot of pain in the meantime, or spend a lot of Plagues as they come out.
The half-damage shield is powerful, but requires some dedication and skill to pull off. Immaterial creatures of course can't be halved, since Thorn Carapace cannot stack with Skull Buckler, unless Dual Shield card idea is implemented.