I think it’s community (and lack thereof). When you had to talk to others on your server only, sit in LFG and shout in Ironforge for a UBRS group, then all schlep to Blackrock Mountain, then spend an hour battling through the instance, that was a shared experience with players you’d see more or less often. Now, it’s get in a random queue, get teleported there, race behind some tank who can solo the whole thing, then everyone drops at the end before you can even say thanks.