Before the v3.1.0 specific tweaks, the base expansion introduced several game-changing systems available on the Crusader Kings II Wiki:
The Pope was made less likely to grant excommunication requests for landless characters, and the AI for Byzantine Electors was improved to prioritize candidates with high Martial or Intrigue. Crusader.Kings.II.Holy.Fury.Update.v3.1.0-CODEX...
Event troops for the Children's Crusade now correctly disband once the war ends, and a higher morale penalty was applied to reflect the "army" of peasants and children. Before the v3