I really like how Second Life works, and I don't think anything that came later really replicates the things that are really special about it to me, even though people will tell me that the reason SL isn't super active is because other things replaced it. (Though OpenSimulator probably counts given that it's supposed to be mostly compatible.) It's far more than just "3d chat with user-generated content".

I really like that Second Life is this big vast world. The stuff I build has a location - it exists somewhere, alongside things other people made, and my house is right next to a house someone else made, next to a public street that provides structure and context that my house fits into. There are these big continents to explore, seamlessly walking from one area to another, and I can fly over a water pathway between my continent and another big one, and fly all the way from my house to the first SL region that was ever made if I take the right path.

Sometimes I like to collaboratively build towns with other people on Minecraft. We'll build our bases near each other, and visit each other and I'll just generally check out what my friends are building. I also had similar stuff in old BYOND games where you just had this big shared map anyone could build on, and you could use preset graphics or bring in your own, for both the map tiles and your player graphics. (And I have my Tilemap Town project which tries to replicate those BYOND games specifically, while drawing inspiration from MU*s and SL too.) I feel like Second Life captures this sort of idea but on a much bigger scale, with so much more freedom to make whatever you want, and be whatever you want. I can make pretty much anything with the scripting feature.

It's really cool that Second Life is so open. I like that they actually publish the source code for the official client and that third party clients are welcome and officially listed on the site, and that you actually do have a bunch of choices, and there's no dependency on a proprietary game engine either. I feel like it's definitely moving backwards to go from that to something made in Unity that prohibits client modifications. (which would be VRChat)

I guess in short I really value openness and big, connected, collaborative worlds.