Well, as long as I've played it - a couple of campaigns, I must admit, it adds:
1) Way more regions and factions than vanilla. The map is slightly expanded eastward, as far as Kwarezm (more or less Eastern Iran). You have way more regions in Spain, Central Europe and in the Middle East. The campaign starts in 1100, and therefore between the new factions there's the infant Kingdom of Jerusalem. The other factions are Kievan Rus', Kwarezm Shahdom, Republic of Genoa (replaces vanilla Milan), The Crown of Aragon, Lithuani, Norway and an emergent Teutonic Order State.
2)With RR enabled your armies will slowly evolve from a feudal levy tipe to a more and more professional stage as the game progresses. Military progresses (plate armour, heavy mail, gunpowder) are now tied to specific dates and therefore warfare is more authentic (no more advanced plate armours in 1200)
3)Area of Recruitment. In foreign regions you cannot recruit your highest tier units, but instead regional units such as bulgarian archers, magyar horse archers, armenian infantry, etc.
4) New traits and ancillaries. Really, really useful for roleplay. And faction leader specific traits (e.g. Crown of England, King's Standard) which will be inherited by your heir when he rises to the throne.
5) Slower development of cities and castles.
6)Improved AI (expecially on campaign map, since it enjoys a lot of bonuses)
7)Expect thougher and slower paced campaigns than in vanilla. Reaching good technology levels is vital in early game, since everybody has levy units.
8) Early and Late starting campaigns. In Late you can play as the Mongols and the Teutonic Order.
If I forgot anything, correct me - I'm a newcomer too
