[Ygorl]

I'm no game reviewer, nor even (anymore, alas...) a big-time gamer, but here's one little half-formed take on it:
The depth of Dom3 comes in large part from magic. Each nation has different magic available (determined mostly by available paths) and the pretender gives you an opportunity to further customize. Discovered independent mages also help to make two games with the same nation different (my last big game in Dom2, as Tuatha, I ended up strongest in Death, while my big Ermorian neighbor was quite mighty in Nature). The magic available to you gives you choices - which schools to research, towards which spells, towards what strategy? Item forging and army-summoning both depend on and expand these choices, as well. Troops have many stats, and many possible effects, powers, modifications, and item slots, all of which are important and all of which are influenced by magic.

It is true that if you took out the magic, you'd have a fairly mediocre strategy game... It would have the added interest of the dominion system (though you'd have to include priests to get that fully, and then you've already got a more interesting game, though still pretty basic) but it would certainly not have the kind of depth or longevity that inspires strategy nuts to play obsessively for years. On the other hand, you could probably remove all the non-mage units and replace them with "footman", "archer", "flier", "horsey", "zombie", and "big monster", and the game would still be interesting. It's depth is obviously not economic; it is tactical and strategic, because of the rich magic system and everything that springs from it.

edit: looks like it's all already been said. Me type slow.