It's not all pre-made skins, either; you can go into Blender or Maya and make whatever you want. (What Fortnite calls "creating skins", from a supply of pre-made parts, Second Life users call "getting dressed".)
The downside is that you have to be good enough at 3D artwork creation and the tools for it to make something good. Anybody can create in Minecraft or The Sims. There's a high bar to entry in the systems that allow full creation capability. This is a real problem when attracting new users.
Nobody else seems to have both "big, persistent shared world" and "fully general user based creation". Most systems limit one or the other. There's also a big problem with Second Life viewers choking on excessively complex geometry, which brings down the frame rate. Second Life badly needs better automated level of detail generation.
(Automated level of detail generation is supposedly a solved problem. The trouble is, the usual algorithms don't work on clothing as separate items. Second Life mesh clothing is not part of the body. It's like real-world clothing, a 3D object with an inside and an outside. Sometimes designers omit the inside, if you can't see it, but usually it's present. Mesh reduction algorithms in common use have a terrible time with thin objects like cloth. If the original has a wrinkle or pleat that affects both sides, which are separate meshes, it's hard to flatten that out during mesh reduction while maintaining the thickness. Nice R&D problem for somebody.)
A sample of things created in Dreams
I would very much enjoy a "Ready Player One" universe with the entrance/lobby of "Wreck it Ralph" where you can jump around (imagine pacman with a fully equipped "Half Life" character ;)
The "buying objects" is the thing that will annoy me, people staying hungry to buy 0s and 1s still doesn't make sense. I understand buying Software, but software is a tool that we use to build something to add value. On a game I don't understand why we need to spend £€¥$100 to "buy" a weapon that does 100 damage instead of 1. (don't tell me demand --> offer, I know what:)
It's more like a filter bubble, where the whole world looks like it conforms to your dreams, and is paying attention to you.