Just had this thought:
It would be really great if you could make tiny notes on opponent's cards. For example, after using Elemental Storm, you know some cards are Wind, even though they're not revealed. If you could just write "Wind" on the card, you could benefit from this. Or similarly you might need to write "Rare card", "Mage", "Unplayed Since round 20" etc etc.
I'm not talking about an essay, just a few words would be fine. If the note is visible directly in game, I think it would get used more than the current note system. (Not that they can't exist alongside each other.)