The Kyoto Protocol's preference for a comprehensive cap-and-trade regime with virtually unrestricted offsets (under auspices of the Clean Development Mechanism and Joint Implementation programs) has been an unmitigated disaster. On the plus side, the European Union learned from the Kyoto disaster by implemented a far more limited emissions trading scheme (ETS), that unlike the international community at Kyoto, paid serious attention to administrative (primarily monitoring and enforcement) costs. Because the EU's approach was more limited, it implicitly recognized that other policies, including tax mechanisms and traditional command-and-control regulations might be required as supplements to the ETS to achieve real, verifiable emissions reductions. Unfortunately, the more sensible and realistic EU approach has been ignored by legislators in the US, who are promoting comprehensive cap-and-trade a la Kyoto, as well as by the same international negotiators who cannot even agree on targets. Those negotiators want to improve, strengthen, and increase the scope of the Clean Development Mechanism and Joint Implementation, which should be dismantled entirely (probably politically unrealistic) or limited to trades in which emissions reductions are real and verifiable.
The bottom line is that until the international community has a workable institutional framework in place for achieving real and verifiable emissions reductions, targets are irrelevant. Pick a number, any number, but focus on what's needed to attain it.