The WaPo's Rajiv Chandrasekaran has a long article on the sacking of David McKiernan, formerly the top U.S. commander in Kabul. The main reason for McKiernan's removal, according to Chandrasekaran:

The decision was not discussed at length within the White House but was endorsed by Obama. It reflects a view among senior Pentagon officials that top generals need to be as adept at working Washington as they are the battlefield, that the conflict in Afghanistan requires a leader who can also win the confidence of Congress and the American public. 

[snip]

"Blame General Petraeus," a senior Defense Department official said. "He redefined during his tour in Iraq what it means to be a commanding general. He broke the mold. The traditional responsibilities were not enough anymore. You had to be adroit at international politics. You had to be a skilled diplomat. You had to be savvy with the press, and you had to be a really sophisticated leader of a large organization. When you judge McKiernan by Petraeus's standards, he looked old-school by comparison." 

--Jason Zengerle