"What is the definition of "successful" for the purposes of this thread?"
My thought would be "validated by attracting reasonable investment money by angels or VC's". In other words the idea would be good enough for someone to believe in it that wasn't friends or family or someone's own money. Of course that's only a starting definition. Someone who did "show hn" and then used their own money but gained either customer revenue or a significant user base would also be a success. (And what is "significant" or "reasonable" is also open to interpretation.)
My thought would be "validated by attracting reasonable investment money by angels or VC's". In other words the idea would be good enough for someone to believe in it that wasn't friends or family or someone's own money. Of course that's only a starting definition. Someone who did "show hn" and then used their own money but gained either customer revenue or a significant user base would also be a success. (And what is "significant" or "reasonable" is also open to interpretation.)