Added this to the list (inserted as #3). I thought of it while working on my next blog post - I had almost forgotten about this change:
Another change that came with the Untamed set was a readjustment of the BCX required to level up cards, which largely benefited max level players and forced Bronze, Silver, and Gold league players to acquire a lot more cards. From Beta to Untamed (Chaos Legion uses the same BCX requirements as Untamed), the number of BCX to max Rare, Epic, and Legendary cards remained the same while the number of BCX cards to get Level 9 and Level 10 Commons actually decreased.
Meanwhile, for every Common level from 2 through 8 (including Bronze, Silver, and Gold league), the number of BCX required increased. For every Rare level from 2 through 7, the number of BCX required increased. For every Epic level from 2 through 5, the number of BCX required increased. And for Level 3 Legendaries (Gold league), the number of BCX required increased (it remained the same for Bronze and Silver league).
Thus max level players had a slight benefit (fewer cards needed to max their common cards) while Bronze, Silver, and Gold league players all had to acquire significantly more cards to level up.