IMO, the President's announcement should do among the following:
1. Lay out a path that leads to Afghanistan's formation of a stable, representative, and broadly-supported government that is widely-perceived among Afghanistan's peoples as legitimate by 2014 (when elections are supposed to be held). If Mr. Karzai could be persuaded or induced not to seek office in that election, the political development process could be facilitated. Political development will be crucial if Afghanistan is to have a sustainable government beyond 2014.
2. Such political development will also need to be coordinated with Afghanistan's neighbors so as to minimize interference should they believe their interests would be threatened e.g., from the rise of a power vacuum, etc.
3. Continue to train Afghanistan's security forces.
4. Troop levels and departures should be consistent with the security requirements of the above framework.
It should be noted that robust pursuit of political development toward an outcome that would be widely-backed by Afghanistan's peoples could erode domestic sources of support for destabilizing elements e.g., Al Qaeda. It could also help marginalize the Taliban.
Beyond 2014, a smaller U.S. force could remain to act as an enforcement mechanism for the political arrangement that would be created until some point in time where Afghanistan no longer needs such a mechanism. The U.S. and Afghanistan's neighbors could ill-afford to see the Taliban launch a fresh offensive and conquer Afghanistan in the wake of a complete U.S. withdrawal as happened in Vietnam in 1975.