AC:B takes everything that was great about AC:2 and improves on it. Everything from the combat to the economics have been tweeked to make it an even more engrossing experience. Ubisoft must be commended for listening to their fan base and taking the suggestions to heart. The devil is in the details, I say. Here are a few...
Combat: Doing battle has never been so varied. In many games, one must memorize an endless array of combos that are gradually acquired throughout the game. In AC:2, new weapons were introduced in a very fluid way. When engaged in battle, countering was usually the most effective means of winning, and things could get a little repetitive. In AC:B, each battle can be unique, and either offensive or defensive tactics work very well. Ranged weapons have been drastically improved with the introduction of the crossbow. Then, the details. Foes toss dirt in your face, grab you from behind, and attack in different manners. One of my favorite aspects of AC:2 was the ability to hire Mercs to attack for me; Now you can hire also order your Assassin brothers and sisters to attack, and their methods mimic Ezio's beautifully.
Economics: I thoroughly enjoyed the ability to earn money in AC:2 and then use that money to improve my villa and surrounding town. Traveling to the villa would take me away from the city I was working in, however. Now, the money you earn is directly invested in Rome itself. I also love the ability to place factions of Mercs, Courtisans, or Thieves throughout the city and countryside. I actually spend a large amount of time and money improving the city before I finished the base missions.
Travel: As part of the improvement process, tunnels require refurbishing. They then provide you an expedient means of travel throughout the city and countryside. Being able to ride a horse almost anywhere you want is also handy, though riding one through the busy streets can be cumbersome. They do allow quick travel across the countryside, however.