I think the idea is neat, and it's pretty fun, but it's way too easy to catch a shape too early and hit it with the shield and have it not bounce off, causing you to lose.
I also really think hitting things with the shield isn't "weighty" enough? There's no screenshake, sound, or particles, and the shapes don't bounce off strongly, so it doesn't feel good to block with the shield.
Overall a pretty good game, though. Music was nice, and the graphics were simple, but appealing. I think with a little more juice and a slower ramp up of difficulty this could be a fantastic game.