Here are some possible criteria folks might use to define a "successful" RPG. I don't necessarily agree with all of them or feel that a product must hit all of them to be successful but thought this list might help fuel the discussion.
- Is there a published physical book or other permanent artifact?,
- Do people know about it if you mention it?,
- Do I hear about people actually playing the RPG and enjoying it?,
- Could I convince one of my groups to play it, if we had the time of course?,
- Does it show up on (and stay on) various online "looking for players" / "looking for games" sites?,
- Can I see games played using this system at conventions?,
- Does it have some type of organized play program?,
- Does the publisher continue to publish material for it?,
- Are other publishers publishing material for it?,
- Is there a healthy community of GMs and creators regularly talking about it somewhere?,
- Does it have some type of open license so other people can build off of it or publish for it?,
- Can those publishers publish for it forever?,
- Are there regular successful crowdfunding campaigns for it from both first and third party publishers?