I have no problem with the fact that those who work harder should have more money in their pocket. But it's not always the case. The salary are based on a multitudes of factors, but it's not always fair.

Life is not fair. In this country, everyone has the same opportunity for life, liberty and the pursuit of happiness. All actions will not lead to the same result. This is what Obama wants to do when he talks about a "level playing field". He wants the outcomes to be the same regardless of the individual's actions. I've said it before and I'll say it again (well, actually, George bernard Shaw said it first <g>):

This philosophy aspires to the highest common factor but results in the lowest common denominator.

Even today in an area of equality between men and woman and in our modern countries, it's not rare to find jobs where women receive a lower salary than men (in similar jobs).

That is llegal here and it is up to the woman who is discriminated against to take the case to court.

Another example of unfair salary: do you consider normal that a baseball player gain far more than the president of the USA?

This is what the market dictates. I consider it very normal that I earn a lot more money than someone working at McDonalds, even though they may very well be working harder than I do <s>.