Restaurants serve food. As such, people expect the restaurant to have a good healthy respect for hygiene. You may have heard people complaining in a very loud voice about the cutlery being dirty, the cup chipped or the plate having a stain on the side. People complain because they expect perfection.
"Waiter, this cutlery is disgusting," shouts the fat man on the middle table.
Of course, everyone is listening and automatically checks to see if their cutlery is clean. It's a little like when someone shouts "Fire!" Everyone runs regardless of whether there is actually a fire or not. People shouting causes chain reactions and shouting in front of the whole restaurant that there are issues concerning cleanliness, really do encourage other diners to question the cleanliness of the restaurant. Reputations are based on the quality of the food. They are based on customers telling other customers about the wonderful meal that they had. Imagine the scandal if the well-known and respected restaurant got a reputation for being unhygienic. Was that man shouting at the waiter justified, or should he have complained another way?
Translate this into how other customers perceive the problem and what happens is that people either think that the customer is loud and badly behaved or that his complaint is justified. If the latter, then word soon spreads about how dirty the cutlery is in that particular restaurant. What the man who complained did not take into consideration before speaking was that how people act provokes consequences.
Look at the couple of potential consequences of this complaint. The restaurant owner could fire the waiter who placed the dirty spoon on the table for not being careful enough in his job. This sounds a little far fetched, but when the reputation of the restaurant is in question, it's a possibility. The restaurant owner may also decide that the member of staff responsible for the removal of items from the dishwasher should be chastised. He may also have heard the complaint before and think about contacting the manufacturer of the dishwasher to see if anything could be done to improve the cleanliness of the cutlery which isn't a bad thing. The worst potential scenario is that other customers complain or spread the word that the restaurant is dirty. That's creates the bad reputation referred to above.
Now look at a reasonable customer. The customer discovers a dirty fork on his table. He understands that dishwashers don't always clean things properly. He also understands that the waiter is a hard worker and may not have noticed a small detail such as a dirty fork. He isn't going to make a fuss but still wants to eat from a clean fork.
He calls the waiter, though in this case scenario is discreet. The waiter arrives at the table, the man hands the offending fork to the waiter and asks in a quiet voice if the waiter can change the fork for another. The waiter will of course oblige and will be happy to provide alternative cutlery which is shone to a sparkle. In this scenario, what the man who complained did was create a win-win situation. He was satisfied by the response of the waiter. No one heard his complaint and he obtained a clean fork with which to eat his dinner. The waiter may even mention to kitchen staff that they need to look out for the odd dirty piece of cutlery that may emerge from the dishwasher.
In the scenarios created above, the least offensive complaint is the discreet one. It embarrasses no one, least of all the guests who are dining with the complainant. Loudness tends to embarrass other diners at the same table. The discretion the last complainant decided to exercise showed his good manners and did not put the reputation of the restaurant into question. In fact, because no one other than the waiter heard the complaint, restaurant guests at adjoining tables were able to enjoy their meal without needing to inspect the cutlery from which they were eating. Loud complaints upset people and are really not productive. Discreet complaints in the case of having a dirty item of cutlery would therefore seem to be the most effective.