One of the major abuses that Luther complained about was the sale of indulgences. Indulgences were certificates that could be purchased from the Catholic Church that promised to reduce the amount of time a person would spend in purgatory after death.
Luther argued that indulgences were essentially a way for the Church to raise money and exploit the fears of the people. According to Luther, it also contradicted the central tenant of Protestant faith, which state salvation was achieved only through faith and not good works.
Clerical Corruption
The Catholic Church in Luther's time was rife with corruption. There were many examples of priests and bishops living extravagant lives even with their promise of poverty, taking advantage of their positions for personal gain, and engaging in immoral behavior.
Luther was very critical of this corruption, and felt that it undermined the Church's mission to provide spiritual guidance and teachings the word of God to its follower.
Authority of the pope and church councils
Luther also challenged the traditional belief in the absolute authority of the Pope and church councils. Luther argued that church councils were capable of error and maintained that ultimate religious authority reside, with the Bible alone which he called sola scriptura