I don't think that Kagan is a stealth candidate and I think that she will undoubtedly be a consistent liberal voice on the coourt. In the same way I believe that President Obama is a mainstream Democrat and will constantly move the country in a more progressive direction.
However, I also know what kind of Democrat Obama is turning out to be. He is the type who makes labor unions accept an excise tax to get healthcare passed but refuses to do much to push for labor's most important agenda items like the Employee Free Choice Act. Obama is the type of Democrat who is pushing for the closing of Guantanamo Bay but who also believes in issuing an executive order to assassinate a U.S. citizen. He is the type of Democrat who consistently gives away liberal positions in hopes of courting conservatives who have no intention of supporting him. Obama is the type of Democrat who signed some of the most progressive legislation in a generation into law but still seems enthralled by the consensus, elite economic opinion that got us into the mess and seems to think that the way out is to inflict pain on the middle and lower income classes.
My uncomfortableness with Kagan ultimately has less to do with her then with the President who nominated her. While I am broadly supportive of President Obama and his agenda, I am fully aware of what objectionable positions and actions he is able to take. I have the same worry that Kagan will be a mirror image of the President in that regard.