A major disappointment.
First, the good. Divine Wind offers a beautiful map with more provinces and enhanced graphics. There are numerous achievements of varying difficulty one can try for. There are enhanced rules for Ming (China) and the Japanese Sengoku period. Options for peace treaties have improved, and rather than provincial decisions, enhancements come in the form of buildings.
All of these offer major promise, but the AI is a critical failure. Given there was some emphasis placed on this, I'm disappointed.
The DW AI fights a war of maneuver: Rather than commit to major battles, it would rather siege your provinces. This makes some sense but is easily exploitable. For example, if the enemy creates a single large army (a 'deathstack') and sieges one of your provinces, you can run around behind it with three or four armies and therefore take its territory four times as fast.
Further, if you do have a single large army you can force the AI to abandon its siege. It does not want to fight a battle at a disadvantage. Again, this makes sense - until you march your army towards the AI, it breaks siege, you stop moving, then the AI stops and restarts the siege losing whatever time it spent up to this point.
Between these two points it's exceptionally easy to gain war score and therefore win a decisive victory.
There is also a problem with cascading alliances. Once more, a fine idea that didn't execute so well, this allows the defender or attacker to bring in allies after the war began. For example: France attacks England. Later on France allies with Scotland and invites them as well.
Here the problem is that local conflicts can easily turn into continent wide melees. In the example above, France and England could both make new alliances and toss them into the fray. Furthermore, if one of the 'allies' is much stronger than they might take the leader's role for their side. This allows THEM to bring in THEIR allies.
In one game I played, one of the early campaigns to unify France turned into a war that stretched from Sweden to Naples (southern Italy)
EU3: Divine Wind has amazing potential - but it will take some time to develop. As of right now, DW offers nice visual candy but worse gameplay than its predecessor, Heir to the Throne.