There are lots of places on the Hypergrid that people have put huge amounts of work into, but that hardly see any visitors.
Of course, one contributing factor is that sims which aren't listed on OSW practically count as non-existent because people will probably never know about them.
But even if Mount Grace was listed on OSW, it would stay unnoticed unless it had three or four permanently-online alts of the owner on it. Most people only gravitate towards two kinds of sims: whatever a big event is currently happening on and freebie sims offering the latest and greatest and hottest avatar accessories stolen from SL. Even sex sims are far behind.
If I were to mention places that I think deserve more attention, I'd recommend those built by a friend of mine, JohnWinston Vandyke (
https://opensimworld.com/user/JohnWinston). His sims aren't places where you can teleport in, keep standing at the landing-point, cam around for five minutes and see it all.
Strawberry Fields is a Beatles museum that leads the visitor through various phases in the career of the Fab Four, basically for the same target audience as the two Pepperland sims, but with a different purpose.
His Star Wars-themed sims are even more spectacular. If you want to visit them, bring a lot of time for each one of them (Korriban alone is a maze that'll take a few hours to see everything) and switch to mouse look.
Also, when you visit one of his sims, turn the music stream on. It's part of the experience and not yet another generic dance or classic rock stream.