Minimum Income Requirements

As you may know, individuals must fulfil a range of criteria before being granted entry to, or leave to remain in, the UK. As there are multiple kinds of visa, these criteria can differ, but common requirements include the ability to support oneself, a good knowledge of the English language and the absence of particular criminal convictions.
All applications are assessed on a case by case basis, according to their individual merits and against the part of the Immigration Rules which relates to the purpose for which they are seeking entry. 
I believe it is fair to have rules on bringing spouses from abroad into this country and on family reunion. As you point out, the Migration Advisory Committee (MAC) has suggested a salary threshold for skilled workers of £30,000. This is to ensure that migrants are making a positive contribution to the UK economy, paying into the public finances and not placing downward pressure on earnings.
Designing a comprehensive new immigration system will of course take time and consideration. That is why the Government has taken on board the MAC's proposals, while continuing to consult employers, devolved authorities and others to determine whether the threshold is at the right level. As the White Paper released before Christmas makes clear, the Government believes there should be some flexibility where skills are in shortage.
The Government also maintains a Shortage Occupation List, which identifies occupations that are in high demand and urgently need positions filled. The Home Secretary has asked the MAC to review the Shortage Occupation List. Additionally, the MAC will provide an annual report on the status of the UK's immigration system.