I remember once reading about failures in customer service through "omission" and "commission" - the former obviously being a failure through not advising the customer of a certain piece of information that would enhance their experience, the latter being what was experienced in the original posting - adding something unnecessary which turns the whole experience from good to bad, in this case the comment about slowing down the queue of Speedy Boarders. I remember flying on BA once and my special meal was booked wrongly. I asked the cabin crew if I could have one of the standard meals as my travelling companion was eating one and there was no reason I couldn't eat it. I was told that he would have to check if somebody didn't want theirs first, but then proceeded to add "I have 300 meals for 300 people, you can't just book a special meal and then decide you don't want it". Commission again!
Unfortunately with airlines looking at cutting costs in all sorts of areas they are taking the subtlety and responsibility away from the check in staff, and unblurring (if that is a word) the line of what can and cannot be done to assist the passengers, turning everything into a "yes" or "no" situation. Try flying on one of the lo-cos with excess baggage these days for example, no flexibility whatsoever!
I do believe, however, that a few simple words and actions can make a lot of difference - a greeting, an explanation in this instance that you were in the wrong queue and unfortunately couldn't be checked in there and of course at the gate not using a sarcastic tone, would have presumably made all the difference. I think people should write about bad customer service, and good service too. And if you write to the airport Managing Director then at least he / she will be aware that there may be an issue regarding service provision with one of the companies in the airport, perhaps leading to passengers not flying that route again, affecting overall passenger numbers etc etc.
XSB