The first game was good, but it used to annoy me to no end that stealth was pretty much nonexistent - more often than not, the game would force you into group combat anyway. And since all you needed was to hold block and counter whenever an enemy attacked, being mobbed by 30 guards was just boring. The game had no feel of challenge to it, all it had going for it was the crowds and landscapes... And the open-world parkour, of course.
The second was much, much better, and I loved it. Ezio was still undefeated in melee combat, but now at least there tended to be fewer guards mobbing you and stealth runs were actually possible, even if I was only challenging myself. Brotherhood added stealth requirements for 100% completion, so it's going the right way, I think. Giving you many options, but strongly suggesting stealth is the best one.
However, Brotherhood also felt like a big bundle of minigames thrown together just to cash in on the success of AC2. The entire segment of the story was unnecessary and only drew out the time you got to spend in Reneissance Italy. About halfway through the game, I felt tired of the setting. I'm with Yahtzee in that I expected the series to be about exploring different historical periods with Desmond as the framing device.
The story... It pulls you through, but it's really simplistic with its extremely black & white conflict and often feels drawn out just for the sake of padding (pretty much the entire Catherina Sforza act feels unnecessary, for example). And one thing that bothers me is that despite priding itself on historical detail, the series exploits lots of the shallow pop-cultural pseudo-historical preconceptions we have. Yes, I can ignore the bullshit "genetic memory" premise, but despite the "Animus simplification" and "Templars rewriting history books" handwaves, it bothers me that we get Hollywood History from a series about exploring historical periods little-found in fiction.
Also, they gave Copernicus an Italian accent. Fuck you, Patrice Desilets.