It varies depending on what type of service do they offer. For casinos, must have is the provability of the fairness of their games and their withdrawal/deposit system. Customer support is a factor too, and if they failed on this side of the service, usually it makes me want to go turn my back and just forget that they exist. Same for all the other services. If they failed on customer relations, they might as well fail on other aspects because they don’t listen to the customers’ concerns and would likely not want to change or improve what’s weong on their service.