I'm still finding the diplomacy deep and engaging... The trick of course is that you have to approach it in an utterly cynical, machiavellian, manipulative fashion. Remember that your allies aren't friends, only people who've agreed not to attack you out of pure convenience, and you have to continue to make it clear to them that they would be better served keeping that alliance and pursuing other avenues of expansion ;)

In my current Russian campaign, I quickly snapped up all the rebel provinces that I could while sending my diplomat to ally with everyone as usual. I ended up with a Polish province being surrounded by my new rebel conquests, which I considered notably inconvenient, but I didn't want to go to war with them, so I used an agent rush to turn it rebel (10 spies, 4 assassins for sabotage, and enough priests to convert the province more or less entirely to Orthodox) and then conquered it.

Of course then I decided that I should really use my newly minted army of (By this point) fairly skilled agents to wreak more havoc... I carefully eliminated the Polish royal family, which resulted in a couple of assassins with more than 10 skill (I also used a couple of Princesses to "convert" two of his generals to my side ;) )

After destroying the Polish faction through espionage, I snapped up a couple of their provinces but left the rest to my Danish allies (And in fact helped them seize one province, earning their gratitude).

I also wanted one of Hungary's provinces (But just the one) and I noticed the Hungarians were acting a little belligerent towards one of my territories, so I moved a huge cav army out of the city and to what I gauged was just outside the visual range of the spy I knew he had in it, which prompted him to send in his army and lay siege.

Unbeknownst to the Hungarian King however, I gave Byzantium a rather sizable 10 turn tribute to sweeten our relations at the same time I was baiting my trap. So when the Hungarians laid siege to me at Kiev, breaking our alliance and effectively declaring war on me, Denmark (Who were still rather fond of me for my assistance in their campaign against the rebels formerly known as Poland) broke their alliance with Hungary, and Byzantium, whom I had managed to achieve "Perfect" relations with, did them one better and actually declared war on Hungary.

I laughed maniacally as the Hungarians fell into my trap, and descended on them with my 1500 man all cavalry army.

My 1500 men crushed his 1800 (But to be fair, my 1500 men were all Druzhina, Boyar Sons, and Kazakhs), and slew his best general (Who was also a family member). I quickly moved this army to lay siege to his castle (I can't remember the name of the province... it starts with the letters Iai I think).

Apparently my crushing defeat of his best general and full stack army spooked the hell out of the Hungarian King, and he sent me a diplomat with a fire under his feet. The diplomat begged for a ceasefire... I countered by offering the ceasefire in exchange for that one little province I wanted, which he immediately agreed to. I probably should have asked for more territory and given some to Byzantium, because my offer was still deemed "Very generous", but I got what I wanted out of it... And I'll keep pumping money into Byzantium so that they can crush Hungary.

I'm thinking that, unlike in the first MTW, I might actually be able to maintain an alliance with Byzantium long enough to achieve an allied Orthodox conquest of the world :D