When a customer comes into the store looking for a new role-playing game, a rare occurrence, I see it a bit like shopping for a new computer. The RPG is the operating system. Dungeons & Dragons is your classic Windows (WOTC really does resemble a mini-Microsoft). White Wolf most resembles Apple. The Indy Press world is certainly the various flavors of Linux. I've always considered Spirit of the Century your Red Hat Linux, a comfortable gateway to open source. Of course, there are many more operating systems in this example than in the computer world, which is a gigantic part of the problem with the industry; more options than consumers. It's spread far too thin. So the most obvious question if you were to buy a computer is what do you want to do with it? What applications do you want to run?
Applications, in my example, are not supplements. That would be the obvious answer, but supplements don't allow you to use this kind of operating system. The applications are players. Your requirements for a role-playing game are entirely dependent on who you'll be playing it with. If you have a group of die-hard D&D players, your choice of something other than D&D is likely to be incompatible. Sure, there are some likely choices based on similarities to D&D or a desire to take a break from it, but your core application of this game system are your existing players. This is also why D&D has an enormous market share in our store, at 75%.
If you walk into the store and desire to play a role-playing game, there is one game with a guaranteed spot, one company that offers a good organized play system for their role-playing game. That's Wizards of the Coast with D&D 4. If you have no applications for your new operating system, there are some "built in" apps that are bundled and available for your use, every Thursday night at our store. We have other games on the calendar, but access is a little harder to obtain, as they're semi-private games. So when a customer walks in looking for a role-playing game, question one is who are you going to play with?