Well for me, the main reason to jump in a new expansion in the beggining is to play the auction house - you can make sooo much gold by farming materials or just playing the auction house (Protip: buy mats in the morning/weekdays and sell them in the evenings/weekends).
You can easily make enough gold for playing for free until you are done with the expansion.
I have played WoW since Vanilla, and stopped a few months after Legion hit. I am not too sure I will buy the next expansion, though. Part of me feels like I am betraying my maxed level characters, but at the same time WoW expansions are getting a little bit stale. :S
I'll wait to see how I feel at launch ;)
RE: Bye, bye, World of Warcraft. Cya in August!