Since a lot of other people have mentioned search capabilities, I will add that the search and filter features of OCTGN are the main reason I use it for spellbook building. With it I can search by Name, which Set it came in, Type, Subtype, School of magic, Mana Cost, Level, Card ID number, whether it takes a Quick or Full action to cast, the Range I can cast it at, what it Targets, how much life or armor it has, what it has in the attack bar, and the Card Text. I'm not saying that the official spellbook builders need this much functionality, but I will say that is what you guys are competing with and it is very useful. At the least I would include the same filters as on the current web based spellbook builder: Name, Type, Subtype, School, Level, and Cost.