There are massive issues that the Court will take up, such as whether the individual mandate is acceptable under either the commerce clause or the necessary and proper clause. There are smaller details, such as whether or not the mandate is a tax or whether the Medicaid expansion is an unreasoble burden on the states.
It's expected that the four liberal justices will hold together in a bloc. Souter, Ginsberg, Sotomayor and Kagan will vote to uphold the law. Conservatives, then, are worried that they'll be able to peel off any of the others - Kennedy is considered the swing vote, but there's concern that Roberts or Scalia could break as well.
No matter what, the Court's opinion will come out in the summer, which might have a huge effect on the 2012 election. Whether or not the decision is acceptable to the American people, it's going to be seen as either an affirmation or rejection of President Obama's health care law.
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