Yea the first one was like only about killing people. The second one is more about helping others, following people, a lot of side missions, puzzles and more. ^^
I am sure that in Assassin's Creed 3 your going to play as Desmond for like..the whole time.
Yea also...Ezio's personality is more enjoyable then Altair's... Altair's always..so serious and stuff. Ezio's...human xD and there's actually a story in Assassin's Creed 2. + Leonardo Da Vinci creates your weapons like dual hidden blades here's the trailer with him if you haven't seen it.