No need to take our words for it, when there's a really nice demo!
The Dominions II demo is more complete than almost any other game demo. 40 turns (most games last 30 - 80), a level 4 of 9 magic research cap, and not all nations are playable... but otherwise, it's essentially the whole game (not patched to the lastest version, though).
The developers have been very quiet, but aside from massively more stuff, a substantially better interface, many game mechanical changes and additional features, I do not expect Dominions III to be substantially different. Dominions I -> II was more like Warcraft I -> II, or Zelda I -> III, or Metroid -> Super Metroid than, say, Master of Orion I -> II or Zelda III -> 64 or Super Metroid -> Metroid 64... in other words, the fundamentals will not change, just everything will be massively better
My speculation, of course.
...and... welcome aboard!