At many blogs (Sullivan, Yglesias, DeLong, among others) you will find ongoing arguments for prosecuting the torturers who ran our government for a while. I am in agreement with the moral stance of these critics but I don't agree with their practical conclusions. I believe that a full investigation would lead the U.S. public to, ultimately, side with torture, side with the torturers, and side against the prosecutors. That's why we can't proceed and Obama probably understands that. If another attack happened this would be all the more true.
On top of everything else, major Democrats in Congress are likely complicit and the Democrats as a whole hardly made this a campaign issue in 2004; in 2008 the economy was their winning issue, not torture.
One of the excellent students in my Law and Literature class wrote the following sentence in his final paper:
In some of the books, and almost all of the movies we have seen that the law goes as far as people are willing to support it.
That sad truth is another cost of the practice of torture. The American public, now having affiliated itself with torture, will be reluctant to condemn torture for some time to come. The "endowment effect" here seems to be strong.
An acquittal or mistrial would lose the chunk of world opinion that Obama has been winning back. And a trial might prompt another terrorist attack, if only to force acquittal and make America look bad once again.
Pushing for prosecution would more likely endanger rule of law than preserve it, which is a sorry state of affairs.
Addendum: Here is more from Matt Yglesias.