The sequel to Brotherhood is like any movie sequel. More of the same good ol' plain fun with Multiplayer added on.
Similar to AC II, the production values is top-notch. There is also a variety of missions though nothing out of the ordinary. Where in AC II, you have Leonardo da Vinci's flying machine sequence, in ACB you have up to 4 vehicles to play with (including again the flying machine). The main difference is that there are more ways to complete the missions, none more so than the ability to call your assassins to take your foes down. You will need to do this if you want to complete the mission fast or lazy to fight the hoards of enemies that will come down on you.
The city building aspects has also been expanded. Instead of a villa, you get to upgrade parts of Rome. There are also shop quests and the game added in some hunt the items activity to fulfil these shop quests. You can also sell this trinkets back to the shops.
The assassin's that you can call upon earlier, can be trained and sent to missions across Europe. It is pretty simple. Just select the mission, keep adding assassins until the success rate hits 100% and wait for 5-20 mins to pass. It could have been better or more involved as I find myself just clicking and sending out assassins whenever I pass by the pigeon coops. Of course you want them to get to the max level so when they're called upon they can survive the fights better.
Speaking of fights, this I felt is one of the better improvements. Borrowing the Freeflow concept from Batman : Arkham Asylum, you can string a series of kills and counter kills and I find it very enjoyable as I try to keep the streak going. The kill animations are varied (some weapons has specific animations) to keep it lively, while the mechanics are simpler than Batman : AA (only 3 combinations to memorize).
There are other things to talk about ACB but I'll skip to the multiplayer component. Ubisoft has done a wonderful job crafting a multiplayer mode that is unique and unlike the traditional FPSes. It needs to be experienced, though you can't use traditional FPS tactics to win the various matches going on now. Furthermore the support (already 2 free DLCs) that continue to expand the MP mode is exemplary, and with up to 50 levels to conquer, you'll be spending more time on the MP mode than the single player game.
To the first-timers who intend to join the Brotherhood, my suggestion is to get and play AC II first which is a classic on its own. Then only join the Brotherhood. You won't regret it.