This was a UX Product Design question I asked a very senior head at HSBC when designing a new application for them. liked the simple answer.
Question: What will make this project successful for you?
Answer: No Noise.
Further: We become invisible. They (users) just use it and the say nothing. There are no complaints. It should just be there.
What the head was saying was that the application just needs to work well. It needs to work to the level that it gets taken for granted. Even though we might feel like we deserve a pat on the back and a round of applause, the best result is silence. Silence tells us that the problem has been solved.