Actually only BC and WotLK were exceptional expansions, everything else that followed was either mediocre or garbage (WoD and Legion). Other games they've developed in the last 10 years are not special either. HotS - mediocre Moba (LoL, Dota and Smite are way better). Mediocre card-based game HS (Magic is still better, while Gwent and ES:L are still in Beta, but they are already proving to be of a much higher quality). Starcraft has been shit for years now. Diablo 3 was absolute garbage at launch, now it's at least mediocre (Grim Dawn and Path of Exile are much better choices). And Overwatch is solid I guess, but only due to no competition in this genre (though if you want a competitive shooter, CS, CoD and Battlefield are better choices as well).

Oh and the content that WoW adds in between expansions is very small, especially since Cata. Not to mention they're sticking to the 12 months drought, and will do that again with Legion. And I'd agree with you that WoW's expansions add a lot to the game, however the last 2 expansions WoD and especially Legion, added a world mass that's like adding 2 Wrothgar Zones every 2 years.

ZOS added much more in 2 years. 8 DLCs against 1 Expansion that's half the size of either BC or WotLK.