AC2 is as close to a perfect sequel as it's possible to get.
Ezio is a great main character, with a likeable personality, a compelling backstory, and solid motivations - all things that the rather two-dimensional Altair lacked. The story in AC1 was intriguing in a general sense, but in AC2 it has become a very personal thing, and is so much stronger for it.
The basic gameplay mechanics haven't changed much, but the variety of missions and objectives put its predecessor to shame. The combat has also been overhauled and is now much more exciting. On top of that you will find yourself utterly immersed in renaissance Italy, with visuals that are as close to flawless as a video game has probably ever come.
Plus, the optional objectives are now interesting and rewarding - there is an economy in AC2, and every objective earns you money to spend on weapons, armour, clothes, furniture for your villa - you name it. The sense of depth is impressive, to the point where you can spend hours simply exploring the world without doing any story missions whatsoever.
In short, AC2 does what a sequel should do, (and what many sequels fail to achieve) retaining what people loved about the original whilst adding improvement, new dimensions and innovations to create an even better experience.