[Endoperez]

You have twice as much gold, resources and supplies. This means that you can recruit a mage/turn, AND lots of troops/turn. This means that you will always have national armies.

Independents and research have also been doubled. Essentially, this means that early conquest isn't quite as easy for pretenders that take lot of buff spells.

Castles were explained already.

Random paths are restricted. Ulm's Master Smiths have 10% chance of getting one of AEFS, in addition to their E2F1, and only Loremasters have full randoms. Most nations have trouble branching out into several paths.

Taxes are automatically adjusted to remove unrest. Manual is a bit faster in that regard (AI never puts tax under 50%), but it's good enough.
You can see and give order to all armies moving into province X at the same time, by selecting province X and pressing 'y'. You can set site-searching spells to be monthly cast, and they'll cycle through all your unsearched provinces (unfortunately, it includes your capital, but what are few gems for less micro?).

In the next patch, militia events will be made rare. New events that give nation-spesific units have been added for several nations.

No more themes; instead, there are three Eras into which nations are divided. The Eras are Early, Middle and Late, and are shortened as EA (for Age) or ME (for Era) interchangeably. EA Caelum before Raptors left shares the same space with EA Niefelheim, EA Helheim and new EA Vanheim; MA has most of the base theme human nations, while LA has most of the "corrupted" and "fallen" themes/nations. In some cases, two or more themes were mixed together to produce LA Marignon with Goetic Masters and sailing, and LA Ulm with Black Priests, stealthy blood mages and Vampire Count national spell. Early has more and Late less magic than you're used to (e.g. 6 vs 5 vs 4 base gem income).