There is a lot of wasted potential in many features, even looking at the current WoW window of Legion - SL; Magetower is wasted, Class Halls are being wasted, Warfronts, Islands, and now Torghast are all being wasted in different ways.
I think the turning point for Blizzard was Garrisons. Because they put a lot of effort into Garrisons and in many ways they were wildly successful, but they also failed spectacularly by depopulating the game world. I think the lesson Blizz learned from garrisons is don't try too hard with new features just in case they don't work out. Instead of learning the lesson of we did a really good job here, but it unfortunately came at the cost of xyz, we should adjust them so xyz doesn't occur next time.

I'm kind of curious as to where this "mantra" is coming from for Blizzard, because it's been pretty constant the past 6 or so years.