I am now on turn 225 of an 1850 campaign as France, and I have noticed some substantial improvements since patch 1.04.3.
The invincible doom stack is gone.
The million man doomstack, which the AI used to create in European wars, and which could not be directly defeated, is thankfully gone. There were ways to deal with it, but until it was out of supply for awhile, it could crush anything in its path unless the terrain was heavily restricted. Now, the AI still forms big stacks, but they can be defeated directly, even in open terrain. It is a matter of planning, and organization and taking advantage of your leaders, but it can be done. While I have lost some battles, I have won battles where the AI will commit as many as 350,000 men and I have able to commit 250,000, in some cases inflicting devastating losses on the AI. The AI does a better job of massing troops in the right areas, attacking on different fronts, and the major European wars have been a real challenge. Wonderful stuff.
AI use of navies and amphibious assaults.
i almost never used to see this. But, in my current game, China sent a large army to take Hanoi by sea, which led to a war with France, and continued to use amphibious landings and there navy effectively during the war. In my war with Austria, i cut off several Austrian held provinces in central Italy by taking Venizia. The Austrians loaded reinforcements on their small navy and tried to send them to those cut off areas. Fortunately I was able to detect, intercept, and sink the reinforcements and their escorts in an enjoyable naval battle.
If the AI is losing the war, they will make reasonable peace offers, which often leads to the short wars that often occurred in this period.
The shortage of leaders was a horrible problem, as you could increase the size of your army but quickly run out of officers to command them. New leaders would appear once every ten years, or if a leader got killed. Now, the game will generate a couple of new officers as your army grows in size, and this may also be tied into your being at war. This has made the game much more enjoyable,
This was an area of the game that was an afterthought for me, needed mainly for getting commercial treaties and casus belli. Your actions, other than declaring war, rarely had consequences. The game is much deeper now.
Consequences of promising local support
Prior to the patch, i would just use this action where ever I had an interest without much thought, and it would almost always substantially increase my relationship over time. This allowed me to choose the wars i wanted to fight.
But now, the action is not immediately available, though it does appear after a few years.
But if you fail to back up your promise by declaring war once it gives you a casus belli, their is an immediate prestige penalty. And if you persist in failing to honor your commitment, you can lose ten percent of your entire prestige score at intervals. If you have promised local support to two different nations who go to war with each other, you are in trouble, as you cannot declare war on either, and will continue to accrue large prestige penalties. This happened in my current game, where I promised local support to both Prussia and Russia, and suffered heavy prestige loss when they went to war, a was that ended indecisively after a year. By the same token, if the AI had promised local support to a nation, and you declare war on that nation, there is a substantial danger that the first nation will declare war on you.
Defensive alliances work in a similar manner. This makes it crucial to review the treaties that a potential victim has with the other nations before you attack them, or you could wind up with a much bigger war than you want.
That said, promising local support is the most effective way of getting a nation to like you, but it comes with a big risk.
Warscore and the peace system
This works much better than it used to. It is much easier to get the AI to accept a reasonable peace offer, and the wars between the AI are often resolved within a year or so. If you are at was with a nation and its allies, and you make peace with the nation you are at war with, or your ally does, the allied nations will also make peace, automatically.
There are other improvements, but these are the major ones I have noticed.