The froth on the right is predictable. Indeed, it is mandated for a middle of the road candidate like Kagan. That's how you drive the public perception/dialog. It worked for Sotomayor, who was as uncontroversial and center/right as they come, yet still managed to get credible news outlets to report on her supposed "liberalness."
Which brings us to the fervent opposition on the Left. Most of this stems from the fact that (a) there is not much of a record about her, and (b) there is not to tell us about how she will vote on key issues. Oh wait, there is (c) there is not much of a record to tell us how she would vote on key issues.
Now if the opposition were stated this way, I could nod agreement. There is a lot of legitimate criticism of this move from the progressive side. e.g. Obama is going to his inner circle. Kagan is a safe choice rather than an ideological choice who could advance the progressive agenda and serve as a counterweight to radical conservatives like Thomas and Roberts (this NYT piece captures these criticisms nicely, including the Harkin quote: "Why do the conservatives always get conservatives, and we don't get the liberals.")
But I'm increasingly seeing an effort to manufacture conservative credentials for Kagan, which is about as silly as manufacturing liberal ones. We really don't have jack all of any record for Kagan -- which is increasingly par for the course. Anyone with any ambition walks a narrow line of trying to show party loyalty on the one hand and keep ideological cards close to the vest for another. That is what the game has become.
So we in the public have not the least clue how Kagan is likely to vote outside of a few litmus-test issues. But she has no record of previous judicial decisions and little in the way of published work. Yes, she'll be pro-Roe, even though she will be very careful to recite the magic formula that every other judge has recited during his/her confirmation since Bork got axed for answering honestly. But where she stands on important issues from my perspective on the actual work of judging remains a mystery. I'll note that some of the most famous "liberal" justices, Earl Warren for example, were actually nominated by Republicans with no indication of their future direction. But more often folks on the bench end up as advertised. Ginsburg, for example, is a classic example of a Democratic pick. Reliable vote on big ticket items, but a "pragmatist" who generally votes conservative on economic issues and has no over-arching legal theory that provides vision for interpretation.
On the "judging Obama" scale, the nomination of Kagan is more of the same. He has never pursued an ideological agenda that would tie his initiatives together -- one of the reasons he has yet to develop a narrative about his Presidency that would give it some direction. He is exactly as he advertised himself: a smart and charismatic guy who wants to chart a "moderate" course (by which he means as non-disruptive as possible) with somewhat conservative views on security and a Clintonian view of economics and the economics of opportunity. Most of the good/more interesting stuff (efforts to get federal agencies to coordinate around key issues and respond holistically, for example) generally happen below the surface and are not perceived. The fact that there is a real effort to try to get the federal bureaucracy working again is not something flashy or that anyone notices.
As I have observed a great deal since February 2009, the burden falls to us who care to make things happen. To which I will add a second observation. Activism on big ticket items like Supreme Court nominees and healthcare is important, but has less value in setting an agenda overall than showing up reliably on a range of issues. That means things like giving money through Act Blue or Moveon (if you are so inclined) rather than to the DNC (when last they called, I said "I'm with the branch of the party Rahm Emmanuel calls '[obscene gerund] retarded']. It means supporting primary challenges within the DNC. It may mean withholding votes in the general election. But if so, as with any boycott, you actually need to tell Democratic party organizers why you did not vote or they will persistently draw the wrong conclusions.
But it also means being willing to pick up the phone every now and then and calling your Senators' offices and the office of your House rep (and, if so inclined, your state reps) and telling them what's bothering you. Every office has a staffer whose job is to tally the number of calls and categorize them. And the Left does a lousy job of calling -- especially compared to the Right.
So I'm not mad at Obama for being Obama. Got over that. A number of Progressives persuaded themselves Obama was progressive -- something most professional progressives never believed (although I will certainly admit to hoping that the ambiguity would work in our favor). This makes them more disappointed now -- particularly over missed opportunities like the Supreme Court nomination. But that's no reason to make it worse than it is. Which goes for Kagan. That I would rather see a strong progressive with a defined ideology rather than a pragmatic jurist with liberal sympathies. But that doesn't transform Kagan into a closet conservative.