What did Martin Luther want to change in the Catholic Church?

Martin Luther, a Catholic priest himself, attacked the penitential system of the Church.  For, he was greatly disturbed by corruption in the hierarchy of the Catholic Church; there were monks such as John Tetzel who became successful by selling indulgences to people, a remission of the temporal punishment still due to sin after it has been forgiven by the Sacrament of Penance.  In other words, an indulgence lessened the time that a soul would have to spend in Purgatory, a place where the souls of those dying penitent are purified from their sins before entering Heaven.

This selling of a "lessened punishment" for sin became a mercantile operation rather than a religious or spiritual activity.  Such corruption Luther disapproved of, as well as the corruption of power with the Pope and other hierarchy of the Roman Catholic Church.  Luther questioned these points, but was disciplined for his action and his points were not addressed.

In subsequent years, after traveling to Rome and becoming influenced by other forces in Germany, Luther became the revolutionary against the canons of the Catholic Church, among them

• the concept that man can be saved by performing good works,
• the idea that the hierarchy and priesthood are Divinely instituted.  
• the existence of the sacraments of Reconciliation, Confirmation, and Extreme Unction 
• the belief that there is one visible Church established by God whereby men may work out salvation. 
• the belief that there are other sources of faith outside the Bible
• the belief that man possesses free will; what he does, good or bad, is designed by God.