This is one of my most awaited games since I joined GoG, as it has been for others. For me it took the game Civilization to a whole new level. Civilization seemed like a big game with so many different units, but this game really blew my mind. The reason being:
Two mirror worlds to conquer. You have to travel between these using abilities of heroes, spells or traveling through towers linking these two worlds. One is similar to Earth the other of a more magical nature. Makes the game feel like it has a bit of the 3D tic tac toe about it, since the players can have cities on both planes and need these towers or spells to travel between them.
Even more so is the spells. There are more than 200 spells. At least there are 6 types of magic, and Chaos alone has 40 spells. There is several kinds of spells as well. Some for summoning units, giving bonuses to units or cities, giving damage, even terraforming. Like raise a volcano next to an enemy city and watch its population drop and less resources to get.
Since there are several races, I am guessing more than 100 different units as well. As well as several different heroes that can wield items, and you can even create your own magical items with spells. All of this being new concepts in strategy games.
The combat system, where you have city walls, use magic, and use unit and hero abilities. The combat system in Civilization was automatic and boring compared to this. However there still is the option just to press auto for the combat.