BRUSSELS – The most important economic-policy decision of 2013 might well have been taken in November at the Third Plenum of the Chinese Communist Party’s Central Committee, which pledged that the market should be given a “decisive” role in guiding China’s economy. Because China is now the world’s largest exporter after the European Union, and accounts for about half of global growth, decisions taken in Beijing could have a more important impact on the world economy than those taken in Berlin, Brussels, or Washington, DC.