It is generally considered a courtesy for a newly inaugurated President to overlook any sins of the past incumbent in the interests of ‘looking ahead’ and in the knowledge that the same good manners will be repeated after his/her own end of term.
The most extreme example of this was Gerald Ford’s pardon of Richard Nixon after the latter’s ignominious resignation. Arguably necessary for the health of the nation after its ‘long nightmare’, it was nevertheless handled badly, and Ford paid a price by becoming a one-term President.
Not to trivialize Nixon’s crimes (and part of the flawed pardon process meant there was no allocution, no admission of guilt, no mention of the many crimes besides Watergate), the three articles of impeachment  mentioned the actual break-in, cover-up, including the payment of ‘hush money’, misuse of the FBI and IRS, ignoring subpoenas, spying, and such matters (remind you of anyone?).
For these ‘crimes and misdemeanors’ he undoubtedly would have been impeached, but he resigned to avoid that outcome, and was pardoned for any of his actions which might have crossed the line into illegality. However one might have wished to see Mr. Nixon brought to account, no one argued that the pardon itself was illegal. The matter was handled constitutionally, and people got back to their lives and the nation tried to move on.
But what of Mr. Bush? It could be argued that his domestic crimes far surpass anything done by the Nixon Administration, and while some saw Nixon, Kissinger, McNamara and others as international war criminals, there again Bush has far surpassed Nixon in the number and nature of crimes against humanity, if not the numbers of dead. It’s hard to keep track once you surpass a million corpses. 
And yet in Bush’s case, impeachment has always been ‘off the table’ due to the peculiar spinelessness of Nancy Pelosi and the Democrats, and their willingness to sell out justice to gain power that they’re too servile to exercise in any meaningful way.
Having stated that he would request that his new Attorney General investigate whether crimes were committed during the previous Administration, Mr. Obama seems to be distancing himself from that view, as if investigating the most serious crimes known to man would somehow constitute a distraction. From what? And isn’t that what Mr. Bush said about an investigation into 9/11 even as the public clamored for one? An investigation into America’s greatest terrorist attack would distract from the war on terror, said Mr. Bush (or was it Inspector Clousseau?). 
Indeed there would be a lot to investigate. Domestic spying on a colossal basis. Domestic illegal detention and torture. Misuse of the FBI, ICE and other federal organizations. Ignored subpoenas. Illegal signing statements. Treason.
The AG would be kept busy on the foreign policy front also. Kidnapping (Italy has warrants out on 22 CIA operatives with regard to one case alone). Illegal detention. Torture (which we know with certainty was planned at the top) . Vile torture, possibly the lowest indulgence of which humanity is capable, embraced with zeal by an Administration which uses terror to fight imagined terror. (Given a preponderance of evidence, many of us do not believe that 9/11 was concocted in a cave, but perhaps in boardrooms a lot closer to home.)
Then there is the matter of two illegal invasions. Attacking Afghanistan had nothing to do with capturing Osama bin Laden, who it seems is more valuable as cave/bogeyman on the loose, and everything to do with establishing and protecting an oil pipeline. There has been precious little rebuilding but plenty of indifferent collateral damage. One woman who lost her home and her entire innocent family was called a beggar by American officials and was ordered off American embassy property in Kabul. And there has been plenty of innocent fodder for Bagram, Guantanamo Bay, and the many secret dungeons to justify the unjustifiable and bogus ‘war on terror’.
The invasion of Iraq was as absurd as it was brutal and criminal. The MSM didn’t report what went on in Fallujah (indeed, America seems now in the business of murdering journalists who simply do their jobs) , where almost no structure escaped severe damage. Water and power infrastructure were deliberately destroyed. America did what it falsely accused Saddam of doing in Kuwait—it kicked seriously ill patients out of a hospital in order to make room for potential American casualties. It bombed others. American forces prevented ‘military-age’ males (roughly 12-65 years old) from leaving the city and then declared a ‘free-fire zone’ on anyone remaining. They used white phosphorus and thermobaric weapons. They used snipers against unarmed civilians. They killed, and killed, and killed.  It was like Poppy’s ‘Highway of Death’,  but worse. Congratulations, Junior, you finally outdid the old man. Not even animals were spared.
A new day needs to dawn in America, and that is what President elect Obama has promised. But that can’t occur if the past is not acknowledged and reconciled. A harmless skeleton or two left behind in a White House closet is one thing. But in this case the rattling of bones could drown out Mr. Obama’s eloquent voice and poison his Presidency. If it’s indeed time for change, then it’s time to stop pretending that America can do no wrong, and to bring criminals to justice, whoever they are.
 Nixon Articles of Impeachment
 Casualties of American Recent Invasions