Assassin's Creed 2 is a continuation of the first game, picking up exactly where it ended: Desmond Miles finding secret writings on his wall that are a foreboding of what's to come. Instead of Altair, Desmond now takes control of another ancestor, Ezio Auditore, and now roams through the streets and over the rooftops of famous Italian cities.
The good: the improvement in graphics is well-noted, as faces are given more characteristic, animated movement with expressions that make you want to empathize with the characters a little. The control is also a bit better, but suffers a little with the camera moving on its own at some points.
Story-wise, it's a lot more cryptic than the original, almost Dan Brown-esque with how things play out.
The introduction of puzzles when glyphs are found pulls the game away from the slightly monotonous tasks Ezio has to go through and gives the brain something else to do, not to mention reveals another piece of "The Truth".
Treasure chests and feathers have replaced the flag "adventure" from the first game, but have other purposes, like uplifting your uncle's villa, buying better armour to protect yourself with, and help your mother out of the funk she's fallen into.
Assassination contracts are also a great addition, as each one requires something different to be done. Some aren't all that easy and require a bit of thinking.
Races, beat up events and courier assignments: these also keep the game interesting, and there aren't that many of them to do so each assignment doesn't bore easily.
The not so great: monotony. Once you get the hang of what all of the symbols on your map mean, you'll more than likely follow whatever pattern you've set yourself to ensure you've gotten everything: climb a tower to a viewpoint, seek out the treasures and feathers, do other misc. tasks. Rinse, repeat. If you meet a guard, run away or kill him, loot corpse and hide it. Repetitiveness can make even the once-fun things become a chore.