It is questionable whether the United States can succeed with a counterinsurgency campaign against the Taliban, no matter how many more troops are sent; the experts on Afghanistan that I know are divided on that issue. It seems unarguable, however, that such a campaign will be excruciatingly difficult if international forces are expected to simultaneously repress the Taliban and sort out a central government that is at prolonged and perhaps violent war with itself. A loyal opposition to Karzai questioning the election’s legitimacy would be one thing, and bad enough; a dysfunctional split or open revolt would be another.
From what I understand, the coalition military effort and the civilian capacity-building effort have been terribly disjointed, with the civilian effort terribly under-staffed and poorly executed -- Registan has been all over the issue, though you may want to start here. Some of it is the usual aid and development issues - focusing on large infrastructure projects rather than smaller, more responsive and sustainable projects. That in turn has to do with military objectives -- building Western-quality highways is seen as a way to reduce the danger of IEDs, even if the money could be better spent on dozens of community-based projects that bring a greater degree of sustainability, as well as credibility.
Part of it is also a simple, and appalling, dearth of people qualified for the myriad training jobs that are needed there. Whether through the State Department or some other group, it is really inexcusable to not have a base of specialists who can train Afghan civilians to become self-sufficient. That basic training is the bedrock, really, of success in that country -- the military effort is there as a guarantee of the civilian component.
The extra fissure of a government that will not be seen as legitimate makes the situation that much more delicate.