AC2 is where they decided to forgo forcing you to be stealthy in order to avoid getting yo ass whupped out in the streets and sort of just gave you the power to master the combat and basically dispatch damn near anybody who wasn't a Templar from early on. You could slaughter whole patrols and just jump up to a roof top and dip compared to you having to earn back your weapons/skills in AC1 which made you, in my opinion, respect the capabilities of the character until you unlocked another part of him that added to the way he battled.
Ac2, if I'm not mistaken added the banking system and other features I can't remember that did push the series in a better direction from 1's annoying 3 mission types multiplied 30 times around the map, but once I saw they gave you the option to just straight up cleave dudes head in the street, it was a wrap for me in playing that game the way I thought it should be played.
Plus as much as I didn't care for Altair, Ezio was even more annoying until he actually matured and I think that was the last game where he was older and not much of a stereotypical spanish lothario.