Some of the most fun times I've had in WoW have actually been with a friend or two trying to get achievements and transmog (glamour) from old raids and getting rekt by the bosses because they still have mechanics . There was also mount rewards for a speed run for a couple of dungeons and minions that could be sold. Heroic difficulty raids of the previous expansion still gave a nice challenge when soloed. The Death Knight class developed a sub culture of trying to solo everything up to and including some of the easier current content. I can't wait for the removal of level sync! Hopefully it is also extended to trials, Coils and Crystal Tower.