John Kerry

They essentially are correcting a major flaw in the Bush influenced government giving the President very strong powers.

The United States-backed and guided Afghan constitution that was put in place a decade ago created a powerful, almost king-like authority for Hamid Karzai, the man hand-picked by the George W. Bush administration to run the country.
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But that's not really the important bit. It appears that Kerry extracted from both men a promise to work towards creating a new arrangement that would end the supreme powers of the Afghan presidency.

Here's the key bit from the UN statement on the agreement: "The agreement also includes the formation of a government of national unity upon the declaration of the final results of the Presidential election."