The vast majority of letters of complaint I have had have said "Terrible person, sack him now", on some occasions followed by letters from other passengers complementing me on how well I handled a difficult situation.
Invariably the person writing in has had either an invalid ticket, or was in 1st class for no good reason. On one occasion BTP were able to nick someone who has assaulted me after I had pointed out their ticket was invalid, as they had then decided to write in and complain.
Whilst acknowledging that not every member of staff in any industry is perfect, quite often the problems people encounter are due to a combination of factors, and the staff member in the customer-facing role has no power to alter the situation, or is applying the rules that the company apply to the situation. Quite often, the problem may be because of something the customer did themselves, or some combination of events totally outside the companies control, influence, or fault.
I'm of the opinion this knee-jerk "Something has happened that has upset the totally smooth flow of my life, I'm going to complain to a higher authority" attitude has more to do with unresolved childhood issues than a real desire to improve the experience for themselves at a later date or other users.
(Or to quote an Australian friend, "You'd whinge if your a*se was on fire and I p*ss*d on you to put it out").
But I'm now going to sit back and wait for all the usual brickbats about how I'm lucky to have a job, and I shouldn't be near decent people with my horrible attitude.