Most of the game play and story line seems incredibly shallow with hardly any depth or emphasis on combat structure since most fights go attack>parry>attack>parry>repeat. The series isn't terrible, it just seems as if they've been throwing in and taking out crap between the sequels which brings me to my burning desire to point out that Brotherhood was lacking too much in some areas while having way too much in others.
Here's what I think...
Assassin's Creed: The combat system actually makes me feel as if I were an assassin, but proves to be too easy in most cases considering the ease and availability of parry and stab. the story actually manages to grip me without having too many plot holes or unnecessary devices. Further elaborating on the combat system, I felt as if it were sometimes lazy and needing.
Assassin's Creed 2: A great game nevertheless. Ubisoft managed to hold my attention for the duration of the campaign, yet it managed to fall apart in some areas. The combat system managed to improve upon itself in some areas, allowing the use of more abilities and disposable weapons giving more flavor and life to something once dead and uninteresting. Not much to say otherwise about a pretty well-rounded game.
Assassin's Creed Brotherhood: This game was sold under the interest of the first multiplayer scenario. Wonderfully executed since it brought something new and fresh to the table, rather than the common stale and overused FPS. The combat flowed wonderfully when competing against other players due to the simple, yet elaborate compatibility involving perks and techniques. Sadly, it still fell short in some areas due to unnecessary perks, being connection dependent, and the some-what linear usage of perks.
On to the campaign. I can't possibly be more empathetic when it comes to stating the unnecessary features and shallow story telling. This game was a letdown from it's previous version overall due to the addition of mercenaries, side-questing, and much, much more. Overall it seems like a rehashed package that they threw together at the last second to sell with the multiplayer feature. I wasn't impressed if anything.
Upon further speculation, the series will either fall apart from here or continue to take form into something wonderful that was built upon the foundation of trial and error, leading use to a marvelous perfection. Sadly, I won't hold my breath. We can only wait and tell.