The economics of different kinds of jobs works differently, I don't think its quite so simple as the 'heart-of-gold-people-with-a-vocation' and 'self-serving-superfluous-fat-cats'situation that gaelic and others describe.
It depends on training, scarcity, risk, who else is willing to do that job, and the impacts of technology...filtered through the good old laws of supply and demand.
So at the bottom end of the salary league you have jobs that could and are done by people willing to take a minimum wage, and without much training or capital - school leavers, recent immigrants without recognised qualifications or capital, parents (mainly mums) who want local, part-time jobs.For example - care, retail, cleaning, childcare, call centers, catering, laboring etc...If you are doing one of these jobs, however long your experience you could always be replaced by someone cheaper and so there isn't much of a career ladder.
Then there are jobs with a big upfront investment in training, or apprenticeship of some sort - traditional jobs with formal barriers - like lawyers, accountants, doctors, and those where you just have to put a lot of low paid time in in the early days to get connections and experience - journalism, policy and politics, the arts.
There are 'craft' jobs that you get better at over time after initial training - teacher, plumber, chef etc..your earning power goes up, but it reaches a ceiling beyond which it can't go much further unless you go into management or train others.
Then there are winner-take-it-all jobs like being Robbie Williams, David Beckham etc.. where one person can earn millions, and thousands of others aspire to that dream but don't make much of a living out of it.
Then there are careers where you take a risk with your money, or someone else's and this is where the biggest money is - financial sector, CEOs etc... At this end the reward-for-risk argument has pushed up salaries and bonuses, but a lot of people are paid too much because as it turns out they were just riding the market not creating real value, or were benefiting because others covered their risks(as with the bank bail outs.
These are not hard-and-fast categories, and new technologies and business models can change the economics of different jobs over time, but I think they are useful to think about why different people get paid different amounts.