Let's consider a car with a conventional automatic transmission including torque converter. When at standstill, it is possible to have the transmission either at "D"rive or "N"eutral. My understanding is that when at "D", the input shaft of the torque converter is spinning. This in my opinion wastes some fuel (because the input shaft of torque converter is spinning but the output shaft is not, and therefore, there is some power loss in the torque converter). Also, in my opinion it should lead to a reduced torque converter lifetime because the torque converter is spinning and slipping unnecessarily. Of course, because this is not a friction device like a clutch, continuous slip should not be catastrophical.
According to this reasoning, one should shift to "N" when at standstill, especially if the car does not have a start-stop system that shuts off the engine at stoplights. Is my reasoning correct? Any idea how great the magnitude of these two effects (fuel usage, torque converter lifetime) is? Or is it actually the case that the longevity of the shift lever or the transmission is reduced so much that it doesn't make sense to shift between "D" and "N"? My expectation is that the torque converter lifetime is probably a negligible effect, but the fuel economy effect may actually save you some money on fuel costs.
Of course, the answer to this question may dramatically change if the car has some new type of automatic-like transmission. For example, on Toyota hybrids you don't want to change to "N" because doing so depowers both motor-generators and thus eliminates the only possibility of charging the high-voltage battery pack. Also, on dual clutch gearboxes, both clutches are disengaged when at standstill, and therefore, there is no extra load on the engine. However, on conventional CVTs, there is usually a torque converter, and therefore, the answer should apply.
Somewhat similar question: Automatic transmission: Is shifting to neutral while approaching stops bad? ...although this question does not discuss the effects of fuel consumption, only briefly mentioning it.