The abuses that were leaked by Bradley Manning are extremely serious and should be immediately dealt with. If Obama neglects to look into them, he could be putting many lives in danger. Also, the situations could just continue and spiral out of control, so Obama need not delay in his investigation.
Government agencies, including but not limited to those exposed by Chelsea (formerly Bradley) Manning, tend to do whatever they can to expand the scope of their own power. The only functional way to ensure that these agencies are not violating the law is through robust oversight, which must originate from the Obama administration.