Things started changing around when people became disposable for content, no longer requiring social interaction to complete most content of the game (exactly the opposite of what an MMO is supposed to be). For example, Group Finder (especially where you're put with random people) is very convenient, but it also removes almost any social interactions you can have. Much of the game's content has been redesigned so that grouping isn't needed, kind of like when Blizz made a pass to nerf the hell out of group/elite quests to all of them basically being soloable. The concept of "server identity" keeps getting thrown around occasionally by Blizz, but that concept died long ago for various reasons (ease of server hopping, sharding/cross-realm, etc.).
The mentality change was that game design shifted from player friction and game restrictions forcing players to be social to making players able to avoid social interactions completely. In the past, you could have really good social experiences, as well as really bad social experiences... however, they were all social experiences that you likely remember even to this day. In the past, you had a reputation to maintain because you were stuck on your server and said reputation would stick with you. Nowadays, aside from my guildies and a few friends, everyone else on my server and my random groups could be NPC's, as there's very little/no social interaction, let alone any impetus for maintaining a reputation for most players.