Under the two step proposal facilitated by the Danes, COP15 is the forum in which an interim agreement will be designed. This will then form the basis for the binding legal agreement to be signed off on in Mexico next year (there is some question as to whether a legally binding agreement could be arrived at in 6 (hopefully) to 12 months (latest)).
This agreement will include specific commitments for each country (that are being negotiated as we write), including emissions reduction targets, investment agreements, technology transfer, clarification on REDD (reduce emissions from deforestation and degradation), commitment to transparent monitoring, etc.
At the end of the day, the best we can realistically hope for then from COP15 is an effective interim agreement to set the stage for the effective legally binding agreement signed next year.
From a scientific standpoint, the only way we can judge whether the agreement is effective or not is to wait and see if global annual greenhouse gas emissions plateau sometime in the next 5–7 years or so. Except for the global recession, emissions have generally been increasing, and at an increasing rate. So for a plateau to occur in the time frame necessary, we need to see a lowering of this trajectory almost immediately.
The sooner the plateau, the more feasible it will be to meet the necessary rate of emissions reductions to have a 50/50 chance of stabilizing atmospheric concentrations of GHGs to a level that does not equate with more than 2 degrees warming above pre-industrial temperatures.
So I would be surprised that they wouldn’t reach an agreement. The question in my mind is whether it will be sufficiently devoid of loopholes and leakage to result in real emissions reductions of the order of magnitude necessary and within the timeframe required.
Unfortunately nature doesn’t seem to care very much for compromises.