You're right that it isn't, and doesn't need to be, the most complicated thing in the world.

But there's a bit more to it than it sounds like. Deck management, customizing flashcards (html/CSS templates), spaced repetition algorithms (including many user-customizable settings), statistics and visualizations.

Most importantly probably are the things they host online. Right now you can freely sync your data between devices, and they have a community portal where people upload decks to share. I'm imagining these will be among the first targets for paywalls (but could always be replaced by the community).