It really is the quickest way to kill a car. When a car is idling it takes ages to reach operating temperature. So the burn is inefficient, acids etc build up at an alarming rate, the catalyst takes longer to reach operating efficiency. Pistons are not fully expanded so there is potentially lots of blow by in the cylinders. A car warms up very quickly when it is driven and a warm engine is a happy engine.
Im sure there are other reasons but these I think are the main ones. Everyone says "don't idle an engine!" but few people can give any detail on why that is when you ask them.
(Diesels are a little different in this respect to petrol - cue HDifun....)