The next area for Lean to make big strides is in Sales and Marketing. Lean is all about growth, the Sales and Marketing folks play an integral part in making sure that the company grows properly.
Too often organizations are broken up into functional silos that are disjointed, rivalistic, and unable to effectively communicate with eachother. This is the opposite of Lean. Instead, businesses should be organized into Value Streams that are focused on their customers. Managing by Value Streams allows for the quickest flow of information and materials possible with each Value Stream establishing its own performance metrics based on that Value Stream's customer's definition of value.