Changing Jobs for Job Satisfaction?

I-love-my-work, who is considering rejoining the IT world after a stint in business, asks: "A molecular biologist with a PhD at University of Birmingham, in the UK, quits his lab position to become a plumber, since a plumber apparently earns twice what he currently makes (~US$42K). How many of you would change careers if given a chance? What factors would influence the decision (money, hours, upper management, a chance to enjoy more of your life)?" What factors would make you seriously consider leaving your current career for another?