Note cards are great, but I think a really big factor with a good spaced repetition system is to be able to essentially pick the best card to show you at any time. Based upon your history, which card you are most likely to have forgotten and need a refresher on, and would be most beneficial to show you next.

This would be practically impossible with a physical system. In addition to the fact that your memory of a new cards fades faster than cards you saw a long time ago; taking the time to approximate such a system would be a large burden that software can just handle for you.