The only thing I know of is that they've removed the pretty town screen that you had in previous HoMMs. There's quite the uproar over that. While that isn't much of a detriment to the gameplay, I suspect it'll impact the immersiveness of the game.
The town screen was replaced with a town window which has a 2D animated view of your town that will update as buildings are added, although not all buildings will have an impact on the town window. Also you won't be able to interact with the town window.