A fair point; but I think Quinto has his work cut just as high - Nimoy's very presence in the film will be a constant reminder to the audience of that other Spock, while it'll be up to their memories (or lack thereof) to conjure up Bill Shatner.
Karl Urban, as the Kiwi who I mainly know as Eomer in LOTR has to convince me he's that plain talking American country doctor who we all know and love. That's the hardest; I've seen many spoofs and fan recasts over the years and while the Spocks were frequently dead-on, and the Kirks a bit understandably hammy, most McCoys have just missed the character completely. (Case in point: Anyone remember that Star Trek
special with Kirk, Spock and McCoy wandering around Earth circa 1999 as various clips played? Spock was good, Kirk was appropriately hamfisted, and the McCoy guy had no idea of how to do McCoy).
Course, Pine is still Our Hero and Leading Man. I'm not saying it's easy for him, but I don't think he's got the hardest job.