The idea of the expansion came up after the heroes of Azeroth dealt with so much external threats like the Burning Legion. In their own words, "so when we talk about what's the biggest threat in this world, is it the titan or is it the dwarf paladin that put a hammer in that titan's head"? As such, the enemy (at least during the early parts of the scenario) will be the opposite faction, a "vast army of world-destroying forces".
Most heirlooms only work in a limited level range in their default form and must be upgraded to work at higher levels. That costs gold, and you’ll need around 30,000 gold to outfit yourself with a fully upgraded set (counting only those that give you bonus experience). You may need to spend some time on a boosted character to acquire the gold you need to outfit another character with heirlooms, but it’s well worth the effort.
But compare that with BfA's lackluster prepatch. Last week, players' uber-powerful Artifact weapons, which they had spent the entire Legion expansion building up, abruptly burnt out with no in-game explanation, no storyline, no quest, no Dad jokes from Khadgar, just a line of text on the Artifact screen saying they had been sacrificed to help stop the destruction of the world--part of the ending quests for Legion.
Players’ relationships with the NPCs around them have been hit-and-miss over the years. One of the original issues with Thrall, the Horde’s original Warchief, is that players began to refer to him as “green Jesus,” because of how infinitely powerful and infallible he was. But he’s been gone since the Warlord of Draenor expansion, and with the death of Varian Wrynn and Vol’Jin at the start of Legion, the old guard hasn’t been there to guide players the same way as it used to be.