The actual work being performed. It requires no skillset and almost anyone can do it.
Actually, I beg to differ. Menial jobs are often more complex than many of your average middle management jobs. The tasks required in a menial job are often multiple and complex. Also, menial jobs often encompass more than just what's on the application. A fry cook might also work the register, drive thru as well as clean the store. An HR manager on the other hand sifts through applicants, makes decisions and types. If the department is large enough, most of her work is delegated to subordinates.
I'd wager that a large number of middle class positions could be learned with little more than on the job training as there's not much work required. People who like to thumb their noses at these low wage workers are of the mindset that more work equals more money. Well, if wages and salaries were determined by the actual work performed and complexity of tasks, office workers and most middle management positions would be near the minimum wage mark.
A last point is this, no, not just anyone can
do those jobs, it's just that they will
hire anyone to do those jobs. There's a difference.