I read an article recently about how overworked game developers can be, so I felt bad and thought it could be potentially helpful to see what people think are their favorite aspects of each expansion (yes, that means WoD, too). I'll start:
I loved the reputation quests in TBC and WoTLK, and I felt the grind made the items more rewarding for me. I also loved that the rep items, if not all of them, were still relevant.