As the other answer explains, race and, especially, gender determines the kind of minimum wage work Ehrenreich does. Her jobs as a waitress and a house cleaner are particularly tied to gendered concepts of what "woman's work" should be.
However, today class discrimination is what most emerges from this book and what is most pertinent to the political climate two decades after the book's publication.
Ehrenreich is acutely conscious of being from a different class than the people she works with. She is well educated, very well-to-do, and a highly renowned journalist with elite connections. As she states from the start, her family worked hard to get to the point that its descendants, people like her, could be afforded the education and opportunities to rise out of the working class and achieve success. She says she has no interest in going back to a working class lifestyle. She is very clear that she is simply briefly descending into this milieu to gather information for her book.
This influences how she interacts with the people she meets on the job. Her class background sometimes causes her to take on a superior or even paternalistic attitude toward her peers. For example, when she works for the cleaning company The Maids, she finds its bemusing and a bit pathetic that her female fellow cleaners are so dependent on approval and praise from their male manager, Ted. She also takes it on herself to interfere and calls Ted to pressure him to give time off to their team leader, Holly, when she is injured.
Ehrenreich means well, but she can come across as seeing her working class peers not as fellow humans she meets across a table of equality but as zoo specimens she observes and feels superior to. (She tries not to do this, but I would argue she ultimately fails.) While both feeling a deep sympathy towards the people she works with, she also discriminates against them on the basis of class, perceiving them as inferior to her. This kind of assumption of elitism on the part of the educated middle classes has, it is said, worked to alienate the classes from one another and widen political divides in this country.