Choices, choices
1. Depth of customization between nations. I love how their are literally features available to each nation that no other nation has. No cheesy little stat rotation (though a little cheese on top is nice!), but honestly unique abilities and attributes.
2. Depth of economics. Blood, gold, magic, dominion, summoning, and more. Each impacts the other but is not wholy dependent on another economy.
3. A real strategic feel to combat. You can specify strategy and tactics but not actual choices on the battlefield. This is great! Almost no other game gives you a true feel of being the general for a military. You are not constrained to use whatever bargain basement salad shooter got through procurement as a standard weapon, you can choose to search out find the military mix that _you_ want for your armies. In fact, you may decide that salad shooters are good weapons, especially against vine men and treelords
4. The gee wiz/that's cool factor you get from the random independents and special sites. I love finding some really cool site and changing my grand strategy because of some plum/lemon that just dropped into my lap (Hmmm, mount chaining, maybe I _will_ persue summoned troops this game).
5. The exciting pace of the game. I get the fun of building up an uber empire, but I get to have fun along the way. One of the problems I have with games like Civ III is the tendancy to make it so you have to churn out the bare minimum of improvements while trying to maximize military and scientific production. In Dom you can ignore spell research if you want to, you can minimize military production (but don't do it too much, or you'll be singing someone else's hymns soon enough), you can build a really cool empire without constantly monitoring your progress versus an optimization curve to determine whether you are still competitive.