What I'm most excited for in Legion is the new transmog system! Its been one of my favorite features since they added it in Cataclysm and its such an important thing in a game like this. The armor you wear, the look of your character, helps determine your characters identity, even if you don't RP.
Currently my entire personal guild bank on my bank character is full of transmog stuff, and my void storage as well on all my major characters.
With the new transmog system its going to be a lot easier maintaining it all, and a lot easier to mix and match new sets and looks.