Well, simply put, people have been complaining since around the middle of WotLK that the game feels dead and boring. The most radical change to the game are flying mounts. I realize they were introduced in TBC - but flying mounts make sense in Outland as it's a shattered rock floating around in space, already in part disconnected from the high fantasy-mood of traditional Azeroth. Taking flying to the old, and subsequent worlds, however, was bad imo. I think that's the main cause people don't feel as much for the game any longer, but without realising it.