Update:

I just received the nicest message from NW in response to my complaint. It was a personalized message (not a canned response) which apologized for the problems, briefly described their maintenance program for aircraft interiors, and promised to forward my complaint to their maintenance department. They also offered a significant number of bonus miles.

And it came only two days after my complaint! I am shocked! The last time I complained to an airline (United seven years ago), it was about a far more serious problem, and the airline ignored me until I complained to the DOT over a month later. (One reason I stopped flying UA.)

I am very impressed with NW customer service, not just because of their response to this incident. I have also found their phone reps over the years to be uniformly cheerful, professional, and helpful. It almost makes up for the junky planes! (and is quite a contrast to CO, where the planes are immaculate but the phone reps are notoriously bad)