Originally Posted by
Destinas
Most of the plot points concerning BfA were built up over the course of the last decade (N'Zoth, Twilight/Black Dragonflights, Azshara, Nightmare, Xal'atath, Ny'alotha, Naga/Nazjatar, Twilight's Hammer, Wrathion - even the Zandalari have had extended lore since Cataclysm). Instead of BfA simply filling time for players until the next expansion (lore-wise), this is actually the end of a few major storylines, mostly including the (known) Old Gods, Nazjatar, and even the faction war.
It feels like a "filler" for some people because they aren't having fun with it, plain and simple. In their mentality, they aren't enjoying the content or story presented, so they're waiting for the next big expansion, hoping it'll be better. This also happened with WoD, since it was seemingly cut short with the arrival of Archimonde again, leading into Legion. So, you feel like it's a filler because you're thinking like it is, when it isn't at all in reality.
In the end, there are no "filler" expansions - there are simply good or bad expansions.