Assassin's Creed Brotherhood
I can't help but feel that this series has been made progressively easier... and this has to be the easiest yet.
There's very, very little sense of any tension while walking around the city. You can literally bump into every Borgia guard and nothing happens to you... aside from being comedically pushed around. It won't provoke a fight or any real danger. Essentially, you're never in any danger while on the streets.
Compare that to the slightly edgier guards in Assassins Creed II who would have a go at you if you bumped into them or somebody carrying a box, etc. Then compare that to the near-constant paranoia of the first Assassins Creed, where if you even walked too close to a guard (never mind bumping into them), you'd be assaulted by seven or eight nutters perpetually stabbing you. I miss that sense of danger.
The fact that you also have a small army of 10 or 12 assassins to call upon when you are in a fight, just makes the whole thing far, far too easy. That's not to say that all of the developments are completely unwelcome. The crossbow brings a new dimension to the tactics... but all the weaponry available to Ezio makes parts of the game redundant: What's the point of the weaker throwing knives when you have the crossbow? Why use the gun when a crossbow is quieter? Why bother getting close to a target when you can shoot them discreetly with a poisoned dart? etc, etc, etc...
I think the series needs a bit of a re-think. Stripping things back to the bare essentials would be welcome, but I doubt Assassins Creed Revelations will do that. Unfortunately, I suspect Assassins Creed III will probably be even more technologically convoluted, assuming we end up playing as Desmond in the present day.