IMO you should offer free subscriptions to people for creating decks - this free content will attract more people to the site and make you stand out and differentiate yourself from competitors like Supermemo or Mnemosyne.
that is good advice, we really do need a lot more content to get users.
Hard-core users seem to obsess about making their own decks, thus the available content is there as a hook or gateway to becoming a more involved user.
Actually the one thing that has put me off using one of these applications is the lack of availability of a library of decks and is probably the main advantage of a web-based approach. I'm not a fan of the over-proliferation of social network-based sites (Halo's law: "Every site expands until it becomes a social network, those that don't are replaced by those that do"), but I think this is the key case where it could really work to both your and your users advantage.
The hardcore users will be more likely to already have bought Supermemo, have lots of Supermemo decks and be unlikely to switch over. I think you could end up with a larger, broader market than that. But I think the concept is good.
The "content is king" meme exists for a reason.