It's time for some critics.

[Zhuangzi]

To echo some of what sector said, I had a think about it and there are a couple of key reasons why I think this is about the best TBS in only 15 years of strategy gaming:

The leadership system. This is genius. Instead of there being weak and strong units like in most games, the leadership systems means that pretty much any unit is viable throughout the game. Wonderful. Black Dragons are probably the 'best' unit overall but are you going to use them with their 2500 leadership? Or do you want to go for Lake Fairies? Or Evil Beholders?

Level design (at the strategic, not battle level) This is especially true in Darion, which is just put together in such a way as to draw you into the game. While this game doesn't have an interesting story, it does lure you with the quest design. I also love the way that enemies move on the strategic view, unlike in Disciples II.

Thirdly, the battle system. I'm with sector on this. I can't remember a more enjoyable and rewarding battle system in a game. The grid is very small (usually) but this makes it like chess. This is a real STRATEGY game.

Oh, and why does everything have to be compared to HOMM 3 anyway? If I loved that game, and then someone made a successful homage to it many years later, I would be ecstatic. I'm still waiting for a real spiritual successor to XCOM, and Master of Orion 2 (i.e. not MOO3, or Galciv 1/2), and Jagged Alliance 2.