It needs to be able to track per-card quantity, and it needs to have a PTCGO-style deck builder. I know I'm not the only person who builds/tests decks in PTCGO before trying to make them in real life. It would be great to be able to import a PTCGO decklist and have the app tell me exactly what cards I'm missing to be able to build the deck. At the very least, it would be great to be able to use an interface similar to PTCGO to make a real-life deck, where the app knows what cards I have and how many of each. Bonus points if it can keep track of what decks I already have assembled and could tell me (for example), "Yeah, you have all the cards you need to make this new deck, but you're going to have to take a Cynthia out of one of your other decks to do it."
What we don't need is yet another app just to track how close you are to set completion.