Ambassador Haqqani resigned in the wake of Pakistan's "Memogate," a scandal surrounding an unsigned memo requesting US assistance to forestall a possible military coup to overthrow the civilian government. The memo, written shortly after Osama bin Laden's assassination in May, requested help from the US in exchange for a wide range of promises that would compromise the country's military and intelligence networks. Though his role in the memo remains unclear, Haqqani's reputation was unable to rebound from the scandal.
Choosing Rehman to represent Pakistan in Washington could present an opportunity to improve relations, or at least communication, between the US and Pakistan. She was educated at Smith College, so her long-time exposure to the US could provide nuance and understanding that is too frequently lacking between the two countries. Her career began in journalism before she entered politics, which means that, in contrast to the standard career bureaucrats whose double speak often complicates US-Pakistani relations, Rehman is perceived as a more trustworthy, straight-talking politician and communicator. Compared to Haqqani she is more ideologically and politically consistent, and is considerably more socially-minded. Within Pakistan this has been somewhat detrimental to her image-- she has drawn harsh criticism for her efforts to reform the country's controversial blasphemy laws.
Rehman's primary challenge will be to convince Americans to continue working with Pakistan's civilian government without taking a hard stance about the military's involvement, particularly in terms of their policies towards the Taliban. Whether or not US-Pakistani relations improve as a result of this new appointment will depend on how well she can strike that precarious balance.