Hmmm. Well, Dungeons and Dragons certainly has staying power for me.
I suppose a depth and breadth of experience is essential, so that there is always something more to do, another corner to explore, another feat to try out.
It needs to be flexible enough to accommodate different styles of play, and allow different people with different styles and interests to work together to create an interesting story.
It needs to have something which makes it always new, and which has both an element of chance and an element of skill so that getting better means your chances improve, but there is still that moment of truth while the dice/cards/… are rolling.
But it also needs to be sufficiently easy to pick up that the time is spent playing and interacting, and not flipping through rule books to try to find minute details. People want to be players, not librarians or lawyers…
Next: my most memorable NPC.