So while the brotherhood itself is the obvious shift, the most important change is the actual pacing of the game. The most common complaint of 2 had been the slow-and-steady introduction. I seem to remember meandering behind Veronica Mars for ages just to exit the Abstergo facility at the beginning of the game. On the other hand, within the period of 12 Brotherhood minutes Ezio has already had sex, had a cannonball eradicate his bedroom wall, ridden on horseback through a crumbling Rome under attack by cannons and footsoldiers, pulled himself up a wall and single-handedly took out most nearby enemies by wielding nearby cannons against them.
While the plot itself is still somewhat vague you're essentially playing the logical extension of the last title, the assassin who has mastered his craft and now seeks to go on the offensive against the Templars.
A few things about Rome. It's enormous, three times the size of the last game's Florence. It's also a patchy mess, stuck under Templar rule. Due to their corrupted decree Rome has essentially come apart at the seams, putting you in the position to invest in the rebuilding of the city in a similar fashion to your treatment of the village of Monteriggioni in AC 2. The area also appears to function as your new hideout in the 21st century - Veronica Mars' Scooby Gang of friendly 21st century assassins have run to the shelter of Italy.
Within the first hour and a half of hands-on gameplay the Desmond plot appears to have more meat to it than it did in previous titles, largely because aspects of Ezio's mechanics have been adapted more fully for Des. You'll take Lucy and vault her off your arms to reach a higher platform, or crawl Ezio-style up a wall to unlock a bridge. From the little specks of this that were shown, the Desmond segments seem to be in opposition to the free-roaming aspects of Ezio's storyline, however it's still a much meatier portrayal of the Des gang.
If the previous games set up a living, breathing historical city then brotherhood homes in on the systems at work inside. Is your brotherhood just a tool in a similar way to the interactive city dwellers of the previous game? Well they're similar, but different; the brotherhood is a broader interpretation of what's been offered before. Your fellow assassins are slightly more complicated than the meandering droves of AI prostitutes. They have more functions, and more individual characteristics. Whether the Brotherhood will offer more to gameplay than any other interactive NPCs is another story, but working in a team seems to give the game more scope than it has ever offered previously.