There are some myths in this game:
etc. etc.
- WoD was a terrible expansion
- Legion was a great expansion
- WotLK was the best expansion
- TBC was the best Expansion
- Vanilla was the best version
- new: BfA will be a horrible expansion
What people fail to do though when considering those aphorisms is to put them in the context of time. When are those statements made and established? That's the crucial factor here.
When TBC was released the overwhelming majority of players were not used to WoW yet, they were still excited about it, almost nobody had gone bored of it; let alone it was unique among other games of the time. Hell, most people didn't even have the time to see all the content of vanilla at TBC release. As a result it's obvious that TBC would be honored similarly to vanilla.
When WotLK was released the situation was more complex. Most people here don't know that but it got a lot of hate from many veterans. The people that were hardcore at vanilla often liked it but a large part of them thought it was a sellout. They thought it made the game more simple and accessible, more than it should be. Most players in total though loved it because they started later.
With Cataclysm we enter an era of extreme change. This is the time of the first big wave of people that got enough of Wow in general got bored of the game in whole, and not of expansions. The expansion wasn't perfect but the technologies were obviously improved in several ways. Still due to that 'time' in the history of the game, the old bored players labeled it average.
Pandaria got lucky because of that. The expansion was again improved technologically but the big picture was that it wasn't either stellar or too different. But the Cataclysm era had aborted all the bored players of Vanilla so now mostly the excited people remained. Pandaria was practically TBC V2. in that factor of time.
WoD got back to the cycle of Cataclysm. While the expansion wasn't the best ever technically it wasn't horrible at all objectively. But it was the time that the era of the players of WotLK started leaving in majority. WoD was Cataclysm V2 in terms of its bad luck in being at an Exodus.
The cycle continues with Legion. While Aesthetically the expansion pleases most Wow players it doesn't really offer anything revolutionary technically or generally. But WoD has aborted most players that got bored of Wow merely because their time has come so it's labeled good.
BfA is Cataclysm V3 and WoD V2 in its bad luck at that. The people that started around Pandaria will now be old and bored of Wow in general; they do not know why, they will blame it on BfA itself, but in reality they left because they simply got bored of the game in general.