What were some of the abuses of the church that reformers complained about?

Some of the abuses reformers complained about:

The power vested in the hierarchy of the church was seen as usurping a role of authority in the church that Christ alone should have.  The hierarchical structure of the church also took away the rightful voice and involvement of the people of the church. 

Reformers were also highly critical of the corruption and greed seen in high-ranking clergy within the Roman church.  This was seen as out of character with Christ and his disciples.
Many reformers (like Hus and later Luther) criticized the fact that Rome only gave communion "in one kind" to worshipers.  The people only received the bread in the sacrament, not the wine.  Reformers saw this as inconsistent with how Christ instituted the sacrament.

The sale of indulgences by the Catholic Church was the abuse that Luther attacked in his 95 Theses which sparked the Reformation.  These were documents that had the pope's authority behind them, which granted removal of penance owed in exchange for money paid by the persons obtaining the indulgences.

Luther's own primary objection was that Roman Catholic theology overall was an abuse against the gospel of Jesus.  Luther emphasized salvation as a gift of God's grace, rather than as something earned by merit -- which is what he saw as the central theology of Rome.