Join the Discord! What we've found so far (subject to further testing):
* Mob/Boss HP scales with your primary stat. Big jumps at certain breakpoints too.
* Gear without your primary stat doesn't affect the mob/boss HP, *except* for heirlooms. A green +Int cloth shoulder won't change HP for a Fury Warrior, but the heirloom version will.
* Potions also count if they're your class's actual primary stat.
* Secondary stats (Crit / Mastery / Haste / Vers) don't count. Neither do Lifestealing, Shadowlands speed/health gems, and so on.
Amusingly, since the above is scaled to benefit new players and semi-punish twinks, a "naked" twink (TW weapons, DF amulet, TWW rings, little else) is doing pretty well compared to a full decked out one.