UO is the game people play to interact with those who are not in their clan/guild/whatever. UO is the game where people role-play as blacksmiths and merchants, selling their wares and services to total strangers, decorating their homes and outfitting them with "rune libraries" to make them more appealing to customers and generate foot traffic.

When I walk around a player-built town in UO it feels lived in, something I have never been able to say about any other MMO I've ever played. In this regard, UO is more like Minecraft than (World of) Warcraft.