First off I want to go against the prevailing opinion that the first few hours of the game are boring. I found them just as interesting as the rest of the game, and the little twist at the end of the prologue was completely unexpected to me and very well done I think.
The story, overall, is well done and much more focused than previous games I think. Connor has a more specific goal in mind than other protagonists in the series, a definite end goal that he's working towards.
However, character wise Connor strikes a balance between Altair's blandness and Ezio's overwhelming charm. He has a ton of character, to be sure, and he's well represented, he's just not as likeable as Ezio and he doesn't really accomplish as much in the long run.
The gameplay itself is superb. I was a little disappointed at the removal of the button = body part mechanic that was so unique in previous games but all in all it was probably for the best. Combat is a much more active affair than in previous games, you don't automatically block all incoming attacks and counter with always lethal effects, you must actively keep an eye on your enemy's attack patterns and press a button to block at the right time. In addition, the different enemy types that are immune to the standard "block, counter, they're dead" tactics are a nice change of pace. In addition, the Desmond parts are no longer just a boring walk about, and seeing him fight for once is nice.
If there is one thing that is an overwhelming disappointment it is the last 15 minutes or so of the game, and the last 5 especially. The final assassination is supposed to seem dramatic, and is actually carried out via cutscene, but the annoying chase sequence beforehand sucks all the joy out of seeing this final foe vanquished. In addition, the out of nowhere plot resolution on Desmond's end leaves much to be desired, although it does seem to be a good set-up for a possible Assassin's Creed 4, which despite this game's hiccups I would glad