I think they did an even better job with the Brotherhood single player than with the one in AC2. Rome is absolutely packed with content, and the content itself is very well done. Even the little side missions like assassinations or courtesan asignments are book-ended by cut-scenes and have a direct relation to the overall story, and there are tons of them. Another major improvement is the focus on actual stealth during missions. There are plenty of missions where remaining hidden is actually a requirement now.
Combat is a lot better now as well. Enemies are more aggressive, and they will break your guard very quickly (you can usually block the first 2 swings of a combo, but the third generally gets through). This makes counter-kills and dodging a must. The great thing about the counter-kills is your ability to quickly follow up on them with a chain of instant-kills. Very quick and very satisfying. All in all the combat isn't really harder, but it's a lot more fun and faster.
Later in the game you get the ability to recruit your own little death squad, and they are awesome. Sure, it's very gamey to target an enemy, press L2 and have a group of hooded killers instantly jumping from the closest roof top, stack of hay or horses, right into the face of your target, but it's just so much fun to watch. They are also fantastic fighters: 2 of them will easily dominate 10+ guards when they are leveled up. They have access to neat gadgets like pistols and smoke bombs, and they make very deliberate use of it too. They are almost too effective.
I spent an unhealthy amount of hours with the game this weekend, by the way. So worth it.
Ubisoft deserves some mad props for delivering such an enormous monster of a game in just a year (it feels ever so slightly rougher around the edges than the predecessor, but it's really nothing major, and certainly won't spoil your fun). There's literally twice as much content, and they even noteiceably touched up the visuals. Especially the characters look a lot better now.
If you liked ACII, you are gonna love this.