just finished the game yesterday. Felt a lot shorter then the previous ones. I thought the graphics, gameplay were top notch. However was disappointed in the storyline, as again it felt short and the ending was kinda "wtf" and felt cheap (just setting up the next game). Note: when I say "short" it's still much longer than most other games single player modes, but short compared to the other Assassin games.
Haven't fucked with the online p;lay and doubt I will.
BTW, I have no idea what you guys are talking about in regards to the Den Defense. I only had to do it once in the entire game. Not sure why that was maybe I did something differently (I did practically ALL the side stuff before doing much of any of the main storyline).