Love the challenge! Suggestions:
1. Change the fill from stripes to dots or make the stripes thicker, or change the way the cards are highlighted when in a set (probably easiest) because it's impossible to tell at a glance what's shaded and what's empty when in a set.
2. ELIMINATE COMIC SANS. That's just a general suggestion, but that applies here, too. Even Impact would be an improvement. Try some open-source typefaces, there are lots of them out there.
3. Add the "challenge" tag.