Traditionally, the President puts a lawyer buddy in as Attorney General who then can fill the various federal prosecutor slots with allies. That initial AG will then leave after the president gets re-elected and someone with more law-enforcement chops can take over.
Thus, Eric Holder's waiting until almost the six year mark of Obama's presidency to step aside is a rarity. He'll stay on until a replacement is named, which would likely take us beyond (or at least very close to) the mid-term elections. A cynical view would be that he kept unflattering investigations from going forward until the last election in the Obama era is held and is bailing out now that that task is finished.
A less cynical view is that six years of flak from the president's critics is more than anyone should take; that level of duty should earn him a nice position somewhere in the liberal apparat as his next gig.
On a number of issues of contention, the Justice Department has been rather pokey in looking into problems with the administration, with the Fast and Furious gun case being the most notable, making Republicans have a slow and furious burn over their lack of transparency on things.
Will the new AG be more forthcoming? One would hope that would be the case.