Nethog, if I had to choose one reason why to buy Dom3 (after having played Dom2, that is), it would be taxes. Checking and setting taxes in Dom2 was for me the most boring thing in the game, in Dom3 you can (and usually do) let the taxes be set automatically. That is a bless in itself
Then there are of course the new nations, new units, new spells, better UI (then again, just because the UI in Dom3 is better than in Dom2 doesn't actually mean the UI of Dom3 would be that super
). The fact that you don't have to choose before the game starts what sorts of forts you build is absolutely great too, you choose where you want to build forts, as this makes a difference
The pretender designing phase is even more of a game than it was in Dom2, because of the option to take a dormant/imprisoned pretender; the decission to take or not take an awake pretender changes your strategy a lot.
I can't say if Dom3 would for you be enough value for $55. For me it has most definitively been