I accidentally reread the story last night. Huzzah! I've been reading a very long (990 pages!) short story anthology and there it was: "Tangents" by Greg Bear. It was even better than I remembered. Not only does it have the super-interesting multidimensional world and people, the characters are excellent. They're all underdogs: there's a brilliant WWII war hero persecuted for being gay, there's an unwanted Korean-American boy genius, and there's a female aspiring writer who takes care of them.
My point is, an unforgettable story has a great big-idea plot as well as empathetic well-drawn characters with internal arcs.
As writers we should aspire to that in all our work.
Are there any unforgettable stories stuck in your mind?
Good luck writing your own unforgettable stories!