Pros:
Game-play is beautiful. Free running has been improved, and exploring the Frontier region is the highlight of the game thus far.
The fighting system takes some getting used to, but is more fluid, and less button-mashing (relies more on combo timing, and remembering which particular attacks are useful on which particular enemies).
The map/level design in general is really well done. Boston and environs are gorgeous, and the Frontier is fantastic.
The hunting/trading system is a nice touch, even if it doesn't make a lick of sense to be given 500 pounds for a bear skin by some dude in a covered wagon, heh. In general, though, I like it, though at this point in the game it is too early for me to say how useful it is for anything but idle amusement between killing folks.
The story line so far is pretty good, managing to avoid outright "AMERICA FREEDOM RAR!" nonsense. Quick notes: Spoiler
Cons:
There are some annoying graphics bugs that pop up during some of the non-cinematic cut scenes and game-play. This may be due to me having an older PS3, but all of my software patches are up to date ...
The enemy AI is about as bad as it always has been. During combat, they actually acquit themselves well, and are pretty good at trying to attack you when your back is turned and whatnot. However, they are really stupid in a lot of other situations (especially when you assassinate one of a group ... there seems to be some sort of conflict between line of sight awareness and the "stealth" afforded by assassinating instead of outright attacking, which sometimes causes enemies that see you assassinate someone to freak out, unable to decide whether to become alert and attack or to stay unawares, and they will sometimes just walk in circles).
They made "escaping" from a fight a lot more difficult in the city (in the Frontier it is pretty easy, you just climb a tree and go). Unlike previous installations, potential hiding spots no longer appear on your HUD radar, which makes planning a route impossible. The enemies are also faster with better line of sight. There are lots of places to hide, and cool dynamic options like windows opening that you jump through, but they aren't marked or indicated in any way - so you have to basically luck upon them, because you don't have time to carefully search while running. Similarly, once you are in a hiding spot, the indicator that you are hiding/blending isn't as clear as it was in previous games, and there have been occasions where I was spotted by an enemy while blending for no apparent reason. It combines to make fighting easier than escaping.
The menus are difficult to navigate (for example, the window that lists challenges, quest achievements, items/collectibles, and that sort of thing is hidden in a submenu of the map instead of the pause menu with the game progress information. Very odd choice).
Sometimes the mission goals (and optional goals for full synchronization) can be confusing. For example, there is a mission where you are given the optional choice to kill a certain character, without having been given any indication that completing a different optional choice before you kill him makes it impossible to do so. Sometimes you don't know about an optional goal until the indicator flashes on the screen that you failed to complete it.
Some story elements were confusing, even if overall the plot is pretty solid - I admit, though, some of my complaints might be alleviated by later developments, it is still early in the game. Quick notes: Spoiler
Also, I get the impression from a previous thread that not many people here are fans of the franchise. So if you aren't going to comment on the game itself, please don't waste everybody's time with, "HUR DE HUR ASSASSINS CREED IS TEH GAY"