The entire thing is made even worse by the fact that locations within areas you're not allowed to visit are shown on your map (although it doesn't say that you're not allowed to visit them). If I can't go somewhere in a game, I need to know that before I spend my time barging my way through crowds, evading guards, and generally making a nuisance of myself. Taking 10 minutes to get somewhere only to be told once you've arrived that the game has arbitrarily decided you can't enter is very frustrating.
Now that the big bad is out of the way, let's take a look at the good, which is just about everything else. Rome is a huge city in the game and you can spend hours just running around it, doing sidequests for the various factions you encounter (thieves, mercenaries, and courtesans). Each of these factions also has a set list of things that they would eventually like to see you accomplish (just because it'll impress them), things like hiring each group a set number of times, silently killing guards from behind or from a hiding place, etc.
The game also adds in the ability to recruit new assassins and to send them abroad so that they can complete quests for you (ones that you're not allowed to go and do). The assassins gain experience, can level up, and earn you money by completing these quests. Assassins can also be called in to help you in your missions, which, unlike hiring thieves, mercenaries, or courtesans, doesn't cost you any money (though the assassins may lose their lives).
As you are in Rome and not your home town in Brotherhood, no longer can you earn money by improving your villa and making its surroundings look spiffier. Instead, the majority of your income is derived from paying to reopen shops around Rome and investing in landmarks. Depending on how much you have improved the city, the bank gets a set amount of cash every 20 minutes which can in turn be used to buy weapons, medicine, equipment, and for opening other shops.