Since the opening of the official Russian Heroes V site and forum by Nival Interactive, the developers have answered a number of questions submitted by forum members in this forum thread. Perhaps the most interesting confirmed feature is the random map generator, which many Heroes IV players missed. It was also confirmed that the underground level and some known heroes would remain.
In general, these answers give some specific examples of the game's overall shift back to Heroes III, confirming the removal of such Heroes IV features as caravans, alternate dwelling build system, distinct melee and ranged attack types, and armies without heroes. To view an English translation of the Q&A, click here. The translated version will grow as more questions are answered in the official thread.