I'd agree that salaries/wages is a poor way to measure the distribution of wealth. It's a good way of hiding just how much wealth the rich have and the gap between wages and productivity.
The average productivity per hour per person is $60/hr/person. With a 40 hour work week, that's $124,800/year/person. Now, some of that has to be reinvested into new technology, maintenance, and R&D, but the point is that the median wage is far behind the average productivity. They were close until they started to diverge around 1970.
The reason why it is so hard to quantify classes is that there are multiple phenomenon going on, each on continuous scales:
Education: A good education is costly and has a significant effect on the wages people will earn later in life. However, it is not as costly as an entire corporation.
Nutrition and environment: The poor usually eat more processed and less nutritional food due to costs and availability. Stress and other environmental factors are also worse. These factors play a role in neural development.
Relationship to the Means of Production: The working class derives little to no income from investments. The middle class derives a portion of their wealth from investments in stocks, particularly for retirement. The upper class has enough stock ownership to be able to live on: think of how Romney thought that actually having to sell some of his stocks constituted financial difficulty. The upper class also guides the investment decisions, affecting the lives of a great many people. The middle class can be self-employed, or have small businesses, but don't derive most of their wealth from surplus value of the people they hire and labor in those small businesses alongside their employees.
Inherited vs earned: Most people must earn a wage to live. However, inherited wealth can give people a source of income which they did not earn.
Political power: Many poor people don't even have the ability to vote, due to not being able to take the time off to do it. The middle class is capable of pooling money to make changes. The super rich can hire armies of canvassers, buy ads, and make enormous campaign contributions.
Personal connections: Particularly in the upper class, personal connections to people in privileged positions allows themselves and their families to receive preferential treatment for jobs, university admissions, etc.
So, there are varying types of economic privilege, all which kick in at different levels. Personally, I feel it only benefits the ultra-rich to pit low-wage laborers against high-wage laborers. It causes the low-wage earners ignore the ultra-rich and the high-wage earners to fear becoming low-wage earners and ignore that their surplus value is also being extracted by the capitalist class. At the same time, the high-wage earners should be mindful of the status they have over low-wage earners and work to bring the low-wage earners up to their level. Some of these privileges will never go away unless we question the wage system itself.