1) A world that is fun to be adventuring in and interesting to players. As a side bar this includes a breadth and depth of material so you aren’t stuck in any one place. Space 1889 is an example of a game that mechanically was average at best but because the world was so much fun players enjoyed the gaming experience.

Without this the game will fail.

2) Support for the DM. Most of us have day jobs and families. Unless you have the luxury of time for some reason DMs need as much help as possible. If the game requires too much from the DM in terms of campaign building, campaigns will stall.

3) Mechanics that are consistent with the world as described in the fiction. If I’m playing Star Wars and people start running gun toting Jedi because that’s mechanically better than using light sabers - that’s a fail.

4) Mechanics that are consistent and easy to understand. THAC0 is the poster child for a dis-intuitive mechanic that smart people couldn’t grasp because it ran negative to every other mechanic in the game. (I know people defend THAC0, but if it was so good why has it been abandoned and no other game stile the concept?)