Bonnie and I have been going back and forth on whether Obama and his administration should investigate the Bush administration. She may not have suggested that prosecutions be Obama's first act in office (and I didn't think she had), but the political point, and the lesson that can be drawn from the initial Clinton stumbles in office (LAX haircut, gays in the military, etc.) is that a president doesn't have full control of his own agenda. One couldn't simply announce an investigation and hope everyone ignores it for a year or two—it could quickly come to crowd out more important agenda items.
I get the idea that Obama (or the people around him, like Clinton vet Rahm Emanuel) understand the importance of not allowing marginal issues to consume the political spotlight (welcome, Senator Burris).
All of that said, Bonnie's right that the Bush administration has stretched, deformed, and in some cases possibly broken the law. And she's correct that future presidents need to be constrained. I'm just not sure that such investigations are the most efficient way to accomplish that goal; and I am sure that as a practical matter, Barack Obama has a whole other agenda (fix the economy, get out of Iraq, etc.), and pursuing it means keeping secondary issues clear.