HoMM4 introduced several "firsts" that fundamentally changed the franchise's identity:

Adds 6 campaigns, new artifacts, and the "Archmage" hero types.

Combining two different skill trees unlocks a unique "Prestige Class" (e.g., Barbarian + Nature Magic = Beastmaster).

There are 5 main magic paths (Life, Death, Order, Chaos, Nature) plus Might.