Not sure I agree you can role play in AC even because Ezzio, yours and mine, will end the game in exactly the same state and having made all the same decisions. That's still the key. Even something like Bioshock that has leveling (via colecting Adam and not XP), inventory, customization and even some choices doesn't rise to the level of an RPG because you've functionally got IIRC 2 choices (the little sisters and how to deal with the artist). Bioshock and AC have a rich story but when people talk about Hawke not being "their" character that's not true. Ezzio and the dude from Bioshock aren't your characters (that being the point in Bioshock) because you have no/very limited control over what they do.
....
I said RP, not RPG. They are completley different things if you look at it in a certain perspective. In AC I can Roleplay Ezio by refusing to kill Gaurds for example. It doesn't make it an RPG, I'm just roleplaying. Like a play for example. RPG's and Roleplaying are different things; IF you look at it, at an angle where it makes sense.
When I say Roleplaying I mean acting a Role. RPGs have that because it's a requirement, but it's not the the fundamental rules of an RPG.