Grownups understand that a Supreme Court justice’s politics are by necessity a crucial factor in how he or she goes about interpreting the law, since difficult questions of legal interpretation are inherently political. Certainly conservatives understood this when they opposed the nomination of Harriet Miers: Their primary objection to her had nothing to do with whether she was “qualified” for the position, but rather with the fact that, just like Kagan, she had practically no public record. The argument for Miers came down to the claim that conservatives should simply trust George W. Bush to make these kinds of decisions. This is exactly the same argument that political progressives are now being asked to accept in regard to Kagan: that they should trust Barack Obama.
The most central, relevant similarities between Kagan and Miers is that neither have ever been a judge, neither has much legal experience to speak of, and neither has published hardly anything scholarly that would allow you to determine what kind of judge that they might end up being. The similarities might end there, but when it comes to the relevant issues when selecting a Supreme Court Justice, those are some monstrous similarities with extreme significance.
In what relevant ways do they differ? The most important difference has nothing to do with the nominees themselves, but with the president that nominated them, and their respective parties. That is, it was Bush's own party, the Republicans, that derailed the Miers nomination. Conservatives understand the potential dangers of nominating someone with no judicial, or public legal record of opinion. And Conservatives are inherently distrustful of leaders (even one of their own), and thus saw no need to trust Bush's judgment on the matter.
The Democratic party, on the other hand is, by definition, more trustful of leaders in general, and especially one of their own. And even more especially the anointed one Barry Obama. The Democrats simply do not have the gall to insinuate that Obama would ever make anything less than a perfect decision. Neither do they understand the potential problems associated with electing someone with such a complete lack of a record, and partly for good reason. That is, considering that so-called 'unknowns' most often turn out to be quite liberal in their decisions, progressives figure that they have little to fear. While the opposite was true for conservatives; they had every reason to fear that Miers 'unknown' status would translate into her becoming a liberal justice.
So the different repsonses to two nominees who have similar qualifications actually makes a lot of sense given the differences between the parties. I guess the point is that a) the left should be more distrustful of their leaders and b) everyone should be more wary about putting a total unknown in such a powerful position.