Luther felt that the Catholic Church's selling of indulgences was wrong. He did not agree with people being able to physically buy indulgences from the church as repentance for sin. He felt that this directly contradicted the teachings of the Bible
Luther believed that Christians should hold to the teaching of the Bible alone and nothing additional (no additional traditions practiced in the Catholic church).
Luther believed people were saved by faith alone and that this was the summary of all Christian doctrine.
The Bible should be written/translated into more common languages that more people would be able to read and interpret on their own.