A new game has to emulate a genre or a specific work of art (movie, novel, etc) that I love. It also has to have a free (or very, very affordable) and very concise and easy to learn and use quickstart version that can hook me and with which I can run a few stories. If the stories click with me and with my group, chances are we're willing to pay for the full rulebook as well, unless the extended rules get bad reviews or are apparently too complicated.
Also, I have collected a ton of games over the years that I barely played, if at all, and are waiting for their turn, so a new game has to promise something the older ones don't. Not an easy task. (The last games I bought were the CP Red Jumpstart kit, Fria Ligan's Alien, and Sprawl Goons.)