The reason that the leftists have been skeptical to hostile to the idea of Justice Kagan is that she actually seems to have expressed respect for ideas that are not her own. This intellectual tolerance is not shared by most of the Left, nor was it shared by Justice Stevens.
This "tolerance" comes from such radical ideas as daring to allow conservatives to hold teaching positions at Harvard University and a stated respect for Justice Scalia.
But it's been noted by those who know her that there is no doubt that "her heart beats on the Left."
So when you hear someone claim that Kagan "will shift the Court to the Right," demand to know: on what case that you care about will Kagan not take the side that Justice Stevens would have taken? Is she going to side with the originalists on the constitutionality of Obamacare's individual mandate? Is she going to take the side of John Roberts on campaign finance reform?
Even if Kagan is "less left-wing" than Stevens, the Lefties are going to have to point to what that means practically. And the answer is nothing. She'll rule in the same way that Stevens has ruled, and likely will be using the same reasoning. Make no mistake, Kagan is a left-wing nominee and exactly the kind of nominee that Obama wants.