I think a letter of complaint is definitely called fall.

The most successful letters of complaint I have had with United, have been e-mailed to the CEO (e-mail me for the address if you don't know it) and go along the lines of...

"I would like to draw the following incident to your attention in order to give you an opportunity to improve the quality of your company's service....

On a personal note, in light of my experience I have of course decided not to do business with your airline at this time, however if you should feel that the quality of service your company is able to offer increases I would be grateful if you could contact me in order to give you the opportunity to alter my opinion"

That is just about my stock letter and it has never failed me.

[Last time double upgrade + 25,000 miles + promise from the CEO's PA that if I had similar difficulties I was to contact her directly - use her name at check-in when you have a complaint and see the reaction!!]
MF