Pink's book Drive is a complex book that examines over 40 years of collected research into the human faculty of motivation. As a result, any discussion of the "main idea" in a small space will be woefully inadequate, but, gleaning information from an interview Pink gave to Harvard Business Review (HBR) to discuss his book, it will be possible to give a summary of what he thinks is the main idea.
Pink discovered that the cut-and-dry, simplistic notion of reward-punishment motivation espoused in psychological theory has been proven by motivational science research to be incorrect when application is attempted to a broad spectrum of human activities through a broad spectrum of human situations. Pink found that the theory of reward-punishment (do good, get rewarded, do more of that thing; do bad, get punished, do less of that thing), which he calls carrot-and-stick motivation is applicable in menial tasks or repetitive "rule based" tasks like unskilled factory work or other unskilled labor. He suggests that the reason this is true is that, at that level of human activity and that level of human existence, money is the driving factor so needs can be met. Think of Maslowe's needs heirarchy in which one isn't free to consider higher level concepts like selfhood or morality until one has basic needs of living met.
Pink further says that what the research shows is that levels of human activity and human circumstances that are higher than these discussed above--where individuals are at last paid enough so that money is no longer the driving concern and worry--the three primary motivators are progress toward mastery, autonomy and a sense of contributing to that which greater than themselves. In other words, once people are paid enough to live on, they want to master skills and talents, they want to have some degree of autonomous control over their thinking and creative processes, they want to contribute so that they feel part of something that affects people in a larger sphere than own immediate world.
Pink proposes that "incentive schemes" that are often used as motivators while useful in some scenarios more often backfire and cause greater troubles from moral infringements to collapsing the creative process through too narrow a focus which can restrict thought patterns needed in creativity and/or can rechannel motivation into paradoxically demotivating channels. He suggests that the primary first step in motivation is paying people enough to live successfully on. He suggests that the primary second step in motivation is to provide feedback that charts or logs, in some way, individual progress and progress within a larger picture, and he further suggests that this feedback ror tracking of progress be a public presentation at times as well as the standard private presentation in "employee reviews." He further suggests that the primary third step in motivation is to allow significant instances of autonomous activity. An example he gives in his HBR interview is that of an Australian company that periodically allows its employees 24 hours to work on or to create any project or concept they want to, which has resulted in several significant fixes to existing problems and several creative innovations.
In a brief encapsulating summary, the main idea is that reward-punishment motivation is a mistaken concept when applied a broad range of experiences and that true motivators--which become apparent once people are no loger worried about providing a roof over their heads and transportation to work and food on their children's plates and shoes on their feet--are mastery, autonomy, and contribution.