Everyone I've met has a different sort of "method" of looking at the expansions. I look at them in groups of three, almost like trilogies:
The Golden Years
This was wow's birth, and it eventual rise to being one of the most popular games in the world by WotLK. When they still had Warcraft RTS stuff to go off of instead of making stuff up.
Vanilla - Surprising and massive success story. People loved it for reasons Blizzard continues to ignore.
Burning Crusade - Bigger and better, in every way. Set WoW up with some eventual problems though (flying, spectacle creep).
Wrath of The Lich King - WoW is for everyone, even your mom plays it! Mild foreshadowing to catastrophe with LFG.
Decent into Madness
This was when wow started to lose subscriptions consistently, and then eventually reached its lowest point with WoD.
Cataclysm - OY BOY HERE WE GO... Awww you screwed it up. They got cocky and didn't expect people to leave in droves.
Mists of Pandaria - Great expansion that could have saved WoW if it wasn't about Pandas. People can be cruel.
Warlords of Dreanor - Well marketed attempt to save WoW that did almost the exact opposite. Became a lonely facebook game.
The Beast Reawakened
This is when I felt the systems put into WoW finally evolved into something enjoyable (Since Wrath). They found a rhythm and release schedule that worked for everyone. A new age for WoW.
Legion - All hands on deck "save wow" expansion. Redefined old systems (World Quests, Mythic+) and loaded with lore fan service.
Battle For Azeroth - Now that I think they are back on their feet, they want to bring the social parts of wow back (Communities, World PvP), and are kind of rolling back the spectacle creep to prepare for more huge monster expansions. Pretty excited.
TBA 9th Expansion - By my method, the end of the trilogy for this age in WoW. What could happen next? WoW 2? Actual new stuff like combat revamps, system overhauls, VR, or Warcraft 4? The world may never know.
So, how do you think of the expansions?