The first Assassin's Creed was a joy to play. I stated in my review, though, that it felt like a disjointed collection of minigames.
Assassin's Creed 2 ("AC2") takes those individual game elements from the first Assassin's Creed, and remixes them to create a game that's far more entertaining.
For this second game, the developers kept the stuff that works (climbing towers, free-running), and eliminated the things that were less fun (eavesdropping, picking pockets, long combat sequences). You're doing a better mix of stuff in AC2.
There are several indoor locations in this game, and the addition of storefronts on the city streets makes these Italian cities feel much more alive than the dusty, beautiful, barren cities of the first game.
AC2 offers a variety of things for completionists to work at. Weapons and armor to purchase, art to collect from city merchants, Investments to be made in the infrastructure and services of the city around your family's villa. Feathers to collect from high city ledges, and a conspiracy theory to unravel. So many things to do! Bopping between them is great fun.
There are typical side-quests: assassinate so-and-so in service of your mentor, beat up someone's cheating husband, race through the city against a timer, deliver package from point A to point B. Yada yada yada.
However, there are also a few exceptional sidequests in the guise of raiding underground tombs or Templar hideouts. These are masterful puzzles, in which you must make your way carefully through a maze of acrobatic leaps and climbs to obtain a special prize. This style of gameplay in AC2 is reminiscent of Mirror's Edge or Portal 2, in the way it challenges you to puzzle your way through an enclosed area that is reluctant to reveal its secrets. These special side quests each take about 45 minutes to complete. They were a high point of the game for me.
I have two complaints. First, I would have preferred more stealth gameplay, and an option to complete the game without murdering hundreds of innocent city guards. Don't get me wrong; I found it very satisfying to pull an unsuspecting victim off a rampart and watch him fall to his death. But the body count is very high in this game, and the gratuitous murder sprees don't really mesh with the main character's “reluctant hero” personality.
My other complaint with AC2 is its excessive length. The gameplay here is fun, but it doesn't change much over time. What you're doing in the 5th hour is very similar to what you're doing in the 30th hour. I spent over 40 hours in this game, and I was hoping for a finale after 20. This is a rare example of a game where too much of a good thing makes the overall experience less pleasurable, less memorable.
But don't let that stop you from playing. AC2 is frequently on sale for $5, and I can't think of many games that deliver better entertainment at any price.