My main concern is the longevity of the engine and transmission. I see conflicting view points on the internet. Some say changing from D to N and back constantly will cause extra wear on the transmission. Others say staying on D will cause stress on the transmission.
I thought this question was answered under this question, but it has to do with coasting and not while stopped (though some of that answer still applies here).
I think you will continually get a bunch or opinion on this question. The truth of the matter is, an automatic transmission is designed to be left in gear while driving. By shifting out of gear at stop lights, you are creating more wear-n-tear on your transmission/shifter/linkage which would otherwise not exist by just leaving it in gear. When you slow your car down and the engine gets below a certain speed, the torque convertor no longer has enough fluid shearing going on to continue providing direct output to the transmission. It is designed to allow this to happen. Your transmission will not suffer any adverse effects by allowing it to happen. No additional wear occurs because of this.
Bottom line, leave your car in drive and let the transmission do it's work unimpeded.
The only stress on the transmission when left in D at a full stop is the shearing stress applied to the transmission fluid in the torque converter. There is no wear of mechanical components, nor contamination of the fluid when that is happening. The energy is dissipated as heat.
That said, I personally like to put the transmission in N and raise the hand brake at stop lights to stretch my legs and shift my position in the seat.