Assassin's Creed II a must have sequel.
However, I think what really sold me on this game are the environments you get to play around in. Renaissance Italy is a brilliant backdrop for this game. The look of the cities and the politics in the cities are just great. The cities and towns you visit are I feel some of the best I've ever seen. It feels like you're actually in a real city they are so well fleshed out. There is hustle and bustle in them, even in the smaller ones, and there are sounds you would expect to hear as well. It is also infinitely fun free running through and over cities.
Some notable things they fixed which are brilliant ideas and add to the game so much. Pick pocketing, in the first game I didn't even try to pick pocket unless I had to it was a pain. In ACII it's changed to needing a single button. Blending, in the first game blending wasn't too bad but they changed it in ACII to be so much more convenient. You can just walk into crowds of people in blend, which is extremely helpful.
Another huge time waster that is fun is collecting items. There are so many different things to collect and you want to collect all of them. There is just so much to do in the world.
There are just a very few problems in the game that do not take away from the experience at all. I ran into a weird free running glitch and I got stuck on a ladder and launched off a building, not a huge problem, it was more of a funny thing. When guards get assassinated on rooftops you can then loot their bodies. Sometimes when your looting the bodies, they slide off the roofs to the ground below and your still looting the body that's no longer there, again not really a problem especially since now you don't have to go down below to finish looting and reveal you're a killer.
This just barely scratches the surface of Assassin's Creed II. It is an amazing sequel that improves on the first game in many ways. If you want to play a good game with a ton to do, and have a lot of fun while doing it this is a game to pick up.