As a number of people pointed out in comments, I missed something very important when talking about a hypothetical John Kerry administration from 2005-2009, his Supreme Court appointments. It seems very likely that such a Kerry administration would have had the opportunity to appoint four Justices—replacements for O’Connor, Rehnquist, Souter, and Stevens. The result would have been a Supreme Court in which there were six justices to the left of Justice Kennedy, rather than the current situation in which Kennedy is the swing vote.
Instead, by replacing Rehnquist and O’Connor with Roberts & Alito, Bush shifted the court significantly to the right and it seems possible that replacing Souter with Sotomayor shifted the court slightly further to the right. This would have been a very important part of the Kerry legacy, especially because the situation in congress would seem to have made major domestic legislation very unlikely. Another point is that President Kerry wouldn’t have put Ben Bernanke in charge of the Federal Reserve, though I’m not sure what the kind of moderate Democrat who would have gotten the job (Larry Summers?) would have done differently.