It would have to be extremely purposeful, willful bad service. Like someone giving me a bad attitude or being mean or disrespecful. If they’re just stupid or ignorant, I’ll usually give them a pass.
I agree with @Nullo that being on the other side is sometimes an awful place to be, because a lot of customers can be really horrible. My best friend’s boyfriend is one of those people. He’s never enjoyed a meal or service at any restaurant that he’s ever been to in his entire 52 years of life. He is rude to the waitstaff and complains bitterly at every minor thing that is of no consequence. He thinks they ask him for his order too soon, or make him wait too long. They bring water before he’s asked for it, or don’t bring water in a timely fashion. There’s always too much ice or not enough ice. The condiments on the table are never suitable. There’s either too many napkins or not enough and they’re never the right size. Nothing is ever cooked correctly and nothing ever tastes quite right. He is one of those people that makes a big deal out of sending food back. It makes me cringe. It makes me uncomfortable to go out to eat with him and I’ve had to apologize, after the fact, to multiple waiters.
I always try to be super-polite and friendly to service persons and I expect that in return. And I don’t expect service people to be miracle workers. Mostly I expect them to be polite, to know their product and their service (or be able to get correct/accurate information from someone else in the company who does know the answer, if they don’t know immediately) to acknowledge my existence. I expect service people to greet me politely immediately, or if they’re on a call or helping with another customer to say something like, “I’ll be with you in a moment.” just to let me know that they know I’m waiting in line or need to ask a question.
So for me it would mostly be about extreme, deliberate rudeness. Just about everything else, I can let slide.
I’m also one of those people who will write or speak directly to the manager of a company to say something nice about an employee who was particularly nice or helpful. I don’t think a lot of people do that.