Karzai has fallen out of grace with Afghans as well as NATO allies. While Afghans acknowledge that there has been progress made under his reign, such as in the health care and education sector, alot overshadows this now. The government is plagued by corruption, the exclusion of some tribes from power and general indecisiveness. Karzai is seen as a weak leader that would not be in power if it weren't for the allies.
The allies are growing impatient with him for many reasons. He is evasive about corruption caused by opium production and trade, constantly critiques military strikes that cause civilian casualties and threatens to turn to Russia if "America" does not "give him planes and tanks." (Could that be our ticket out of there? Just hand it all over to Russia, they know the country and what to do...)
In our happy war-mongerer W., Karzai had found a supporter that did not utter much criticism. But Obama and Biden have had enough. Open criticism is now on the agenda and it needs to be. If this country shall function, someone needs to be in power that radiates trust and authority. However, this may be hard to find in a land where tribal feuds are hardly forgotten or put aside for national unity and where allegiances can be "rented" but not bought.
The question is, is there really a viable alternative to Karzai? There has to be. Yet, not many serious rivals have declared they will run and there don't seem to be many that are as known and as appealing as Karzai. A consensus seems to be that a leader needs to be a Pashtun with support in Southern Afghanistan. Last year, Obama visited the country and the first person he spoke to was Gul Agha Sherzai, once warlord and governor of Kandahar. The important thing seems to be that a serious opponent needs to be able to be a political professional, but believable enough to "play the tribal game."
Whoever runs for office will have to learn from Karzai's mistakes...and need alot of money.