Nations on the edge of memory. The "oh really?" places. Burgundy, Novogrod, the Quara-Khitai, the Indus Valley Civilisation, the Cahokia Mounds in the United States, the Khazaria, the green Sahara, the tower of the Ghorids. There are so many. The thing about a lost nation or a lost memory is not that it is lost but that it is in the act of being lost, an object becoming a process, showing us the existence of time. What is truly lost, we know nothing of, what we call lost or forgotten is only half-lost and half-forgotten. Just enough remains to let us know that something was there. So when we talk about forgotten realms, we are talking about occluded realms, cast into shadows by time, where we can see times action, where we can see the event horizon of entropy that will ultimately consume us.
Perhaps I see only one side. Our real forgotten realms were not just forgotten, but recalled, re-discovered and found again. Resurrected with research and archaeology. Perhaps the important thing about the real forgotten realms is not that they were forgotten but that we are reaching for them. This sense of new-old or freshly ancient history has its place in our schema of creation. It's not just that this place disappeared, never to be recalled, it's that we are going into it and finding out more about it. The genre may want to be a tragedy but the story is an adventure. The D&D 'story' in particular, finds the future in the past.
(Of course there is a conversation to be had somewhere about what counts as a 'Real' Forgotten Realm and what is merely history. Opinions will differ.)
Then add to that the mood and feel of Greenwoods original, as seen through the stories he told about it in interviews. The feeling of that is elegiac. It's about warmth and friendship and adventure. It's a summery aesthetic. Its a world based on the feeling you get on your summer holidays, where time seems to wheel away without hours and, in Greendwoods case, where you were hanging out with your friends in the woods, pretending to be in another world, and I bet if you were there for a while it did feel like you were almost in another world. (In my case these months were spent indoors, alone, reading bad fantasy novels and playing Baldurs Gate, but whatever).
We do tend to see forgotten kingdoms in an elegiac way. Nobody considers the Quara-Khitai and thinks "Wow, I bet they had to skull-fuck a bunch of people to build that", although they probably did. If the kingdom itself was forgotten then the truculent minority that kingdom was dicking around is twice forgotten.
It brings to mind the idea of a kind of civilisational Elysium where all of the worlds forgotten realms go to exist in the summer of their power and in their best possible selves like Kennedy's Camelot, and Forgotten Realms, when related through Greenwoodss stories about it, does remind me of Ray Bradbury and Louise Bogan and Kennedy on Cinefilm.
The fact that you are playing you is important becasue it's not just about this other world, it's this other world compared to your original world, the world of release compared to the world of constraint.
An aspect of the Summer Realms is that bad things are generally done by villains and are not an inextricable part of the world. It's not that horror doesn't exist, it's that it is defeatable, resolvable and impermanent like summer storms, and that when it is removed an already-existing harmony is restored, a world where it is visible that history does truly arc towards justice, where evil is a mistake, and not part of the design.
But, if there were summer realms there must be winter realms as well, where there is only work, where you spend your time with whom you're told to, where you do not decide what you do, where you are indoors, where you do not see nature or move within it
and where there is no romance, only need.
Maybe the difference between romance and need is between a desire that enjoys its own expression and hopes that it will be returned and a desire that loathes its own expression and believes it will not be returned.