Most sequels are a reaction to the original. They can come to that in a lot of different ways: many movie sequels see that the old movie made some money, therefore the new movie should be the same thing, but more. This tendency is often taken to pathetic extremes. (See, Night at the Museum II, where TWO monkeys slap Ben Stiller in the face, instead of one)
Assassin's Creed II isn't so much a reaction to Assassin's Creed, as a reaction to the criticism of Assassin's Creed. There are the "standard" complaints about AC, and ACII has fixed each and every one of them. You no longer have to wait for ten minutes of dialogue after an assassination, and there are much more assassinations to perform, both large and small.
Let's look at a standard complaint about AC: Lepers could grapple you in an attempt to beg money from you. The game left you little options to deal with them, and the grappling is rather incapacitating. My standard reaction was the ol' hidden knife, and the only other reasonable reaction is just running away. From a leper. Not very heroic.
So what did ACII do? There are still NPC's that do nothing but run up to you and bother you. This time they're minstrels carrying lutes. This time, although they obstruct your travel, they have a sense of personal space, and they give you a little room to operate in. More importantly, there are many more ways to interact with NPC's in a non-combat milieu. These minstrels just want some cash, and now you can just throw gold coins into the air, after which they'll leave you alone. If you're feeling mean, (Which I always am, to minstrels), then you can pickpocket them the instant they get near you. Naturally, after being robbed, they walk away. Problem solved.
Review wise, if you liked Assassin's Creed in general, but had a few complaints, ACII has probably solved them. If you don't like climbing, sneaking, and countering attacks, then it's still not going to be any fun. Simply, Assassin's Creed II is the cleaner version of Assassin's Creed.