Just beaten this, thoughts below:
+More of the same gameplay wise, which is a good thing. I wish more platform games were this forgiving
+Very pleased they retained the settling of AC2 (Renaissance Italy), fascinating stuff and they seem to have learnt to keep the focus on levels in that setting rather than faffing about in the present too much
+Decent performance 60-100fps
+Lairs are improved over AC2 in terms of having a bit more variety, a mix of chases, platforming, fighting etc rather than the excessive platforming of AC2
+Adding the 100% sync goals is a nice touch, gives you an incentive to play outside your comfort zone at times without requiring you to
+Few nice extensions like assassin recruits and a beefed up economic system
+Good graphics
+Strong storyline as usual
-Seems cpu limited as often I only had around 90% gpu usage but sub-100fps, not very well optimised for quadcore either looking at task manager
-Annoying 'home stretch' of missions that are all chained together in Sequence 9, I'd just got the sixth Romulus scroll but hadn't gone to collect the reward as the story mission was really close by to where I exited the lair, this meant I couldn't get the Brutus Armour until after I'd completed the game. Also the option to leave the Animus seems to have disappeared - very annoying as I hadn't got round to doing it yet and had some emails waiting etc
-Borgias boss guy towards the end was disappointing, I just pelted him with throwing knives (first time I'd used them)
-Game length a tad disappointing, only 9 sequences compared to 14 in AC2 and they didn't seem noticeably longer. I blasted through from Sequence 5 to the end today, Steam hasn't tracked my hours but I'd estimate low-20s compared to high-20s for AC2 and I've completed more of the sidequests in Brotherhood. Roughly one third of the viewpoints compared to AC2 and not enough variety of cities, you spend the majority of the game in Rome with the odd jaunt to Firenze etc
-Very annoying map expansion i.e. being blocked from lots of places with quest markers (e.g. Borgias Tower) especially in the south-east corner of the map, often you don't find out until you've run/ridden all the way over there. Also spent ages trying to climb an aqueduct to reach a viewpoint, turns out it is impossible unless you repair it first.
-Assassins felt anti-climatic in the sense that I assumed having done ceremonies for a bunch of assassins that they would then help me later in the game, you know, like NPC fighters helping you out in a big fight or something. But it never happened, so I don't really get the point of it (also very annoying having exactly the same unskippable cutscene for each one, gets boring after the 5th time)
-Bomber mission got very tiresome, as sometimes the steering would randomly stop working and I'd go sailing out of the syncrhonised zone (smacked of console limitations - tiny play area coupled with difficult/dodgy controls is a massive fail. In the end when having to take out two carts I used to deliberately crash to reset if I didn't take out the first one on my first pass.
Overall a good game, now on to Revelations.