Haven players will be able to upgrade low-level troops (e.g. Peasants) to high-level troops (e.g. Knights). However, it costs a fortune.
Sylvan players will be able to select a few most hated enemy units, and their troops will fight better against them.
There will be difficulty levels, and possibly not five like before.
Artifacts are stronger, but are encountered less frequently than in previous Heroes games. There will be no Heroes III-style artifact flood.
Most of the combat AI has been ready for more than a year, but it is still in development - they still have to teach it using tricks like the Griffin special attack. However, it has shown itself a worthy opponent and is praised by virtually everyone having access to the game. The global AI is still under development.
Cavaliers will retain their Charge ability.
There is a multiplayer mode in which players make their turns simultaneously until the first "conflict of interests" - for example, if a hero is within a movement range of another hero, which includes spells like Dimension Door.
The translation of the interview should be available soon.