Genre and same-old same-old: random pondering

Occasionally I see reviews (not my own yet) that say something to the effect of "while this story doesn't bring anything new to the genre..." and it always gets me to thinking. Is it the default opinion out there of reviewers that a genre story should always be pushing boundaries? And if so, when did that happen? I sort of understand the mentality when it comes to sci fi. I mean, really the whole