Editor-in-Chief of Nature since 1995; Beginning summer 2018, he will become Editor-in-Chief of Springer Nature’s portfolio of journals, books and magazines
Chemistry for non-chemists; Entrepreneurism

I found your demand for "most" important a bit of a distraction, so forgive me for ignoring it.

My first thought was "chemistry for non-chemists". Few people write about chemistry for the public, few stories appear in the press. It's intrinsically difficult and, anyway, biology is, for the foreseeable future, just too sensational (in both good and questionable senses) and fast-moving for all but the sexiest of the rest of science to get much of a chance to compete for space in the media. But there is room for unusual science writers who know how to hit a nerve with a neat association between interesting chemistry and the everyday world - there just seem to be too few in existence and/or too little demand. (I'd mention John Emslie and my colleague Philip Ball as two honourable examples.)

The second thought was entrepreneurism. Because of inevitable business secrecy, entrepreneurism too rarely gets adequately opened up to scrutiny and public awareness. That's not to imply a hostile intent - entrepreneurism can provide the basis of riveting tales in a positive as well as negative senses. But, in Europe especially, chief executives of high-technology companies who bemoan the lack of an entrepreneurial culture unsurprisingly resist suggestions that a well-proven journalist be given the chance to roam around their company and write about what they find. Partly as a result of such inevitable caution, and partly because of the way the media approaches business, the public tends to get basic news and oceans of speculation about share prices and profits, gee-whiz accounts of technology, misrepresentation from lobby groups on both sides of a divide, lectures on management, partial autobiographies of successful business people, but, unless a company collapses, nothing like the whole truth. More could surely be done, though the obstacles are daunting.