The recommendations can only be understood in the context of domestic American politics in combination with the (also domestic) material constraints of the forces. The report’s central function is to propose the possibility of bipartisanship in foreign policy. Few ISG members have any knowledge of foreign policy; the painstaking process of identifying muddy middle positions (like the battle reported in today's WaPo over whether it should contain a fixed combat troops exit date (Perry's position) or not (Baker's) makes the exercise pointless from anything but a domestic perspective. The Report basically is more about reconciliation in the US than in Iraq.
Morover, the option of a surge in deployment levels -- as proposed by McCain and apparantely supported in principle by the JCF Chairman Pace study -- was discarded because of lack of troops, not because of inherent operational infeasibility. The proposed draw-down is being depicted in Western media as ‘leaving’: in reality a change of role is proposed, from combat to training.
The reality is, as both Pace panel seems to suggest and as testified by Gates is that the coalition will have their hands full for several, maybe 10 years or more, even if in other functions. The challenge with training is that the Pentagon has not been sufficiently serious about professionalizing its own approach to it. In spite of positive proposals about the necessities of cultural (linguistic, area studies insight) capacities in US forces, very few funds were proposed allocated. As noted in WaPo, there are not enough trained military trainers.
Moreover Stabilization and Reconstruction is still not being taken seriously at DoD. The one-year old DoD Directive 3000 calls for this to change by making S&R ops core competence alongside major combat operations. But as long as combat operations is only major core competence in self-understanding in practice, US forces will not be able to successfully run large S&R ops, whether Iraq or Afghanistan. NATO allies have operational advantages in that domain but are probably too small or too scattered to coordinate and dominate agenda without US assistance. Widespread practice of outsourcing SR training, e.g. police in, has been run ineptly, thus jeopardizing the whole mission. These back ground elements are determining factors in combination with the absence of additional troops.
Pentagon’s ability to embrace and professionally run elements outside combat ops (the rest of DIME), including beefing up area studies and linguistic capabilities, is crucial in long term. But there is not much likelihood anything substantial will change in short term.
All of this, of course, is then again dependent upon general political will to either change Pentagon ways -- or supply serious funding for a civilian reconstruction agency.