What are some characteristics of a fad diet?

A fad diet will typically come from a recent bestselling paperback. It will promise quick results, it may put strange demands upon you (such as zero meat, or zero grain products, etc.), it may entail further and habitual monetary expenditure (lose weight if you buy our product), and it may make strange promises (eat as much as you want and still lose weight). Last, but perhaps most important, it will not be endorsed by the wider medical community.