Global cost of carbon mitigation

With carbon caps in Europe and suggested in the US, and with more and more pressure that the main carbon-emitting developing countries should also join the party, the focus on where to achieve the most carbon reduction for every dollar or euro invested has never been that pertinent.  

A 2007 report by McKinsey ('A cost curve for greenhouse gas reduction') highlights the importance of efficiency in buildings and vehicles, the need to engage power producers and industrial companies with long term incentives, incentives for specific technologies such as carbon capture and storage, and addressing the potential of forestry and agriculture. 

The pertinent question, as always, given that energy efficiency can lead to so much gains, why has this not been done before? (Eg. negawatt power). In other words, which existing (maybe often non-monetary) costs are not counted? 

Based on this report , UNEP Grid-Arendal produced the following global cost curve:

Cartographer/designer/author: Emmanuelle Bournay, UNEP/GRID-Arendal