One such private server, Nostalrius, was a progression server for the vanilla version of the game, meaning the version before any of the expansions were added.
While World of Warcraft still reigns supreme as the MMORPG colossus that it is, disillusioned fans of the game have often turned to other means of playing it when the current iteration of the game is lacking.
This server gained massive popularity, largely fueled by nostalgia, but may have also pointed out to Blizzard that they are losing touch with their audience.
This lead to a massive internet meltdown as /r/wow, the Battle.net forums, and even famous YouTubers such as Sodapoppin cried out in frustration at Blizzard.