If I had to summarize why you should buy Dominions 3 in one word, it would be this: Community.
We have a great community here based on what I've seen so far, and while you don't necessarily need to own the game to be a part of it, it helps.
There's nothing Dominions 2 can offer that can come close to the fun that is in-game diplomacy in a multiplayer Dominions 3 game. Especially if you like role-playing. And, of course, renaming. Assuming you aren't completely anal and have to name your mages 3D1E, you can have a lot of fun doing funny little renaming things in a multiplayer game for all to enjoy. The only thing better than a cyclops crushing your opponent's armies is a cyclops named Ai, I Eye crushing your opponent's armies. Sadly, there is not a mod to give the cyclops a top hat and monocle. Yet.