So what makes this game so fun?

[Sombre]

For me it's modding. There are bucketloads of mod nations now which you can add to the game with relatively little hassle, as well as balance mods, spell mods, graphical mods etc.

I personally spend more time modding than playing and find it hugely rewarding. Not everyone's cup of tea, but I love it. I've made,.. eh,.. let me count,.. Vaettiheim, Ulm Reborn, Jomon Broken, Arga Dis, Tharoon, Skaven, Ogre Kingdoms, Avernum/Exile,.. I might have missed one but that's 8 mod nations right there. I'm also involved in a bunch of other modding projects, and yet I don't do much modding for other games - I have no natural talent as an artist or coder or anything, it's just that Dom3 is so easy to mod for.