Yeah I think Vanilla TBC are in their own cluster, then WotLK to MoP, then WoD to now.It's funny how everyone tries to lump in WotLK together with Vanilla and TBC, when in fact it was the one expansion that transformed the game the most into what it's now during its course. The only thing it was missing from being pretty much more or less the same thing as what came next (besides being the last expansion that could easily build on top of Warcraft 3 story) is Mythic+.
So anyway, Vanilla was an entirely different thing from anything else later on, where they didn't really know what they want to do, and everything was new for everyone. Vanilla to TBC is the biggest change in the history of WoW. So if I had to pick arbitrary points in game where the changes were the largest, the first one would be Vanilla->TBC transition for giving the game a direction (as well as removing most of world pvp besides specified locations thanks to addition of flying), then possibly mid-WotLK for adding all of the quality of life stuff, and then Legion launch for adding Mythic+.
Then again, there were some less important changes that affected some people much more than others. Personally, Cataclysm making 10 man raiding legitimate, and then WoD killing it were two more important points, but that's mostly my personal preference for smaller groups and more personal influence, that clearly not everyone shares.
Expansions between WotLK and Legion did gradually modify game mechanics, the way classes play (e.g. using 2 buttons every a few seconds no longer was a valid DPS rotation), etc., but the changes were not drastic, so it's hard to justify picking a random point and saying it's a new era of anything.
On the other hand, there are some changes that nostalgic people like to pretend changed the game in any significant way, such as cross realm play or LFR.