Nudge is a book and a doctrine. It has become central to the British Government's thinking on behaviour change. The fundamental tenet is that governments should not attempt to change behaviours through coercion, restrictions or regulation but through subtly altering the 'choice architecture' so that people find themselves doing what is best for them or society.
This sounds attractive in many ways but, while it is fine as a rhetorical device, it untenable as as a method of devising policy for two reasons:
1) It begins the process of designing a policy, not on the rational basis of what the evidence indicates is likely to be effective, affordable, practicable and publicly acceptable but by choosing from a more limited set of options; coercive and restrictive options (e.g. tax increases on tobacco) are very often the most effective solution and widely accepted by the public because they see them as helpful in overcoming temptations
2) It neglects the fact that all our behaviours are subject to powerful manipulation by companies through marketing and product design. The tobacco, gambling, fatty food and alcohol industries spend many millions of pounds and dollars exploiting our psychology to sell their products. If governments eschew interventions that protect us against this manipulation, 'choices' are not free - they are just being dictated by profit rather than our own welfare.