Besides the trait system, the army limits and the building slots that have already been mentioned, I prefer Rome I to Rome II for the following main reasons:
1. Skype (or maybe Zoom in our pandemic era) diplomacy. I get why many people found the old system a chore, but I think it was much more realistic. Establishing diplomatic relations was not a simple process and embassy interception was actually a very efficient method of preventing your enemies from forming hostile coalitions. Chatting with a bald guy resembling a 2nd rate actor from a sword & sandal movie, who spouts random tough phrases through a small window, seems stupid.
2. Automatic replenishment. Logistics basically play no role in Rome II. You can attack whatever region you want, without bothering to establish a link between the expedition and your empire. Praetorians can casually replenish in recently annexed Lusitania in a matter of a few turns. Back in Barbarian Invasion, there was a long line of reinforcements, stretching from Mesopotamia Gaul, full of heavy Sogdian infantry and elephants. Also, you no more care about your soldiers. As long as the casualties do not surpass an an arbitrarily imposed percentage, everything will be fine. In Rome I, even if you annihilated your enemy, your invasion would eventually come to a halt, due to battle casualties and garrisoning, unless strong reinforcements were constantly being sent to the front.
3. Distance from capital. It was a great mechanic, affecting your income and public order in the most isolated settlements. It nicely simulated the challenges the larger empires faced. Expansion became more difficult and less profitable, which eventually led to the stabilisation of the frontier, even if the enemy was technically much weaker. In my current campaign as Persia against the Alans and the Byzantines, I am actually bleeding money, despite my outstanding victories, because of the enormous expenses and moderate profits. It doesn't matter much, because my treasury was already immense, but that problem lies with the broken economy not the distance from capital feature. In newer games, they try to implement late-game challenges through very sloppy methods, like Realm Divide, the
unintentionally hilarious civil wars of Rome II and Attila the Cockroach with his ''Lazarus-was-an-amateur-at-resurrection'' Huns.