The only reason I would go back to retail is if they had a great story that I just had to play, bug free.
I'm not much of a fan for WoW PvP, so I don't play it that much. The only story I really liked was Cataclysm and MoP. I'm sure I would've loved Lich king, but I joined WoW in Cata so I didn't get the hype.
I heard there's a lot of changes coming for Legion, so I'm not too excited for that anyway.