Simply put, Assassin's Creed II is a superior game to its predecessor. While it no longer has the 'new concept' feel, a number of additions and subtractions have overhauled it into a much more coherent experience, both from gameplay and story perspectives. The gameplay has a much better flow, a lot more variety and the story is compelling and well delivered.
Sure, the perceived lack of difficulty and variety wear thin at the back, but the compromise is necessary in delivering an accessible but compelling gameplay experience, as opposed to a complex but jarring one such as Hitman. And to top it all off, this is a 20 hour quality single player experience, something a rarity in this day and age. Otherwise, the only thing that's left for the series is a conclusion.