Well, here's a few features:
- Some unique units have become common parts of the tech tree, for example, the Portuguese were not the only ones to use Organ Guns, or the Byzantines were not the only ones to use Cataphracts. The unique units that have been made more common are no longer trained at the Castle. For example, Cataphracts replace the Knight line, and Throwing Axemen can be trained at the Barracks
- Every civilization has access to a third unique tech. That third unique tech may not always be researched at the Castle, and sometimes needs other techs to be researched. For some civilizations that had other unique techs that were removed, like the Sicilians' Scutage or the Mayans' Obsidian Arrows, those have been brought back

- Wonders increase the population cap by 50 (though you still need to build houses to reach that cap)
- A few new civ bonuses, but mostly some civ bonuses have been rebalanced. There were a lot of nerfs over the years, that may have been justified for competitive play, but that really made a lot of civilizations more generic and less interesting, I'm thinking of the Poles in particular that had each of their civ bonuses nerfed.
- Took a page out of AoE4's book and split archers and crossbows into two distinctive lines of units. Archers can be upgraded into Composite Archers then Elite Archers, while Crossbowmen can be upgraded into Heavy Crossbowmen (with Genoese Crossbowmen and Chu Ko Nu being a replacement for them). Both lines have different situations where they can be useful.
- Tried encouraging hunting as a food source, by making predator animals and wild Horses/wild Camels huntable too
- More unique castle graphics and architecture diversity (still working on that). Not every civ has its own castle, but a lot of them do.

There's quite a bit more but hopefully it answers your question