I don't have access to the beta, but here are the improvements I can readily see or have heard about
:
1. Spellcasting is a lot deeper. You can choose memorized spells to sort of customize your tactics in a battle. These units can also learn new spells from places on the map and often have a ritual that can upgrade them to a new tier of power. I really like how this system is looking.
2. The system seems a lot more transparent and user friendly. There seems to be a little shield icon indicating what the defender bonus on certain terrain is, for instance. The making your army screen shows just which commander you are putting your troops under (One of the more confusing things for a beginning in CoE 2, all too easy to have your commander accidentally march naked into a bunch of enemies =P).
3. Subcommanders! I think it'll definitely improve the flow of the game and get rid of a lot of the micromanagement.
4. Some new commanders, and some old ones seem to have been revamped to be more interesting. Though I think the total number is down (16 right now, from what, 22?), I think given Illwinter's track record that won't last for long. The Bergmeister, the Bakemono, and the High Cultist are all looking really cool from what I've seen and heard in the various AARs.
5. Conjecture on my part, but I wouldn't be surprised if the unit count is up by a lot from CoE 2.
6. Trade interface. Nuff said
7. More options in setting up a game, like individual difficulties for enemies.
8. And yeah, the graphics are improved =P