To summarize briefly, this mod includes:
~16 unique factions (eventually up to 23), each with its own sets of unique units, leaders, buildings and often some special mechanic. These factions aren't all complete yet; many still have a lot of generic placeholder units. But each faction will have at least half a dozen unique units, sometimes more.
Factions are designed to play quite differently from each other; Brettonnia is about powerful knights and heavy cavalry, while the chaos warrior tribes have nasty offensive melee units, wood elves are masters of archery and use the power of their woodland homes, high and dark elves have swift moving infantry, dwarves have limited movement but incredible defensive strength and siege weapons, and skaven have swarms of weak clanrats. Araby are at home in the desert and have elemental magic-wielding genies, while Norsca and Kislev prefer the northern tundra. The vampires keep their peasants in thrall and feed on their blood - while the tomb kings are completely undead, needing no food or water, and gaining power from the shifting sands of their slowly expanding desert empire.
While each faction will play in a similar manner each time (with the same religion and unit rosters), different factions are much more unique than in any other mod I've come across; while in FFH a faction might have 1-2 unique units, here they might have 7-8 or more.
A magic system similar to that of FFH, but without "mana" sources.
Basically, there are 8 colleges of magic, 4 elemental magics, 3 elven magics, 3 chaos magics, and unique magics for necromancy, orks and goblins, skaven, etc. Each magic has 3 levels, each of which grants access to 2 spells. So, over 100 spells in the game.
Most mages are specialists in a particular college, while great elven mages are able to learn spells from multiple schools, with valuable synergies.
~7 world religions, all of which have their own unique mechanics and units. Nearly all factions are tied to a specific religion.
An Alignment system of Law vs Chaos; there is no Good or Evil in the world of Warhammer. But civs with similar values are more inclined to cooperate with each other, and to ally against their arch-enemies.
A completely unique tech-tree, with 90+ different techs.
A unit design system that encourages combined arms, and mixed forces (not quite fully implemented yet).
Many, many unique events evoking the flavor of the Warhammer world. Advanced Uprisings, even while at peace, empires may be assaulted by beasts or warriors intent on their destruction.
Named historic heroes from Warhammer's history, with artifact equipment.