Really enjoyed this. It's a very cool concept, and thus far the best implementation of the limitation that I've seen.
It wasn't immediately obvious to me why I could sometimes not play some of the cards, though in retrospect, it probably should have been. Nonetheless, a UI notification "not enough mana" would have removed any potential confusion.
I could see how more card variety (including maybe cards that affect draws or allows redraws or shuffles or something) could enhance the concept even further.
Overall, I don't have much to say. This is just excellent.