Enthralling
Well worked out art (both sounds and graphics), and of course an "inviting" atmospehere. One of the reasons I really like it is that when I was a kid, we never had money to buy such (collectable) cards. The other reasons: fun, challenges, replayability, autosave mode, um... oh and the cards themselves :-D My 2 favorits were the Hound of Tindalos and Mad Experiment!
I really admire the way you planned this out, then made it intricate but easily comprehensible at the same time.
Peace