Many surveys have found that 80-90 percent of people say they are satisfied with their jobs. Recently I saw a reference to one that found a much lower level of satisfaction. According to the Conference Board, "only 45 percent of Americans said they were satisfied with their jobs, which is a marked drop from the more than 61 percent who said they were satisfied in 1987." They gave no detail about their survey, and I wasn't curious enough to spend $395 for the full report, but I found a Washington Post story that gave a little detail. They ask people to rank their satisfaction at work on a scale of 1-5, and the 45% refers to people who rated their job a 4 or 5. So possibly there are a lot of people who pick an intermediate response (3) to this question, but say they are satisfied when forced to make a yes/no choice ("can't complain"). The Washington Post story also said that the survey was based on a mail-in questionnaire, and it's possible that people who aren't happy are more motivated to send it in.
I looked for questions on job satisfaction using a similar format--ones that asked people to pick a number without attaching labels to it. There aren't many. You have to go back to 1984 for a Gallup survey that asked people to pick a number from 1-10. In that survey, 64% chose 7-10, and 18% chose 4-6. The most common response was 10 (24%), and then the percentages tapered off pretty steadily. So it doesn't seem like there are a lot of people in the middle.
PS: The 1984 Gallup survey asked half of the sample a standard "would you say you are satisfied or dissatisfied" question: 70% said yes, and 20% said no, which is a lower level of satisfaction than what is usually found. With both forms of the question, about 10% said they didn't know, which I find surprisingly high--it seems like the kind of thing almost everyone would have an opinion on.