The bad
Not all atheists are hostile to religion, but many do think that religion is bad. Here are some of their reasons:
•    Religion gets people to believe something untrue.
•    Religion makes people base the way they run their lives on a falsehood.
•    Religion stops people thinking in a rational and objective way.
•    Religion forces people to rely on outside authority, rather than becoming self-reliant.
•    Religion imposes irrational rules of good and bad behaviour.
•    Religion divides people, and is a cause of conflict and war
•    The hierarchical structure of most religions is anti-democratic, and thus offends basic human rights.
•    Religion doesn't give equal treatment to women and gay people, and thus offends basic human rights.
•    Religion obstructs scientific research.
•    Religion wastes time and money.

The good
Most atheists willingly concede there are some good things about religion, such as:
•    Religious art and music
•    Religious charities and good works
•    Much religious wisdom and scripture
•    Human fellowship and togetherness

Source: http://www.bbc.co.uk/religion/religions/atheism/beliefs/criticisms.shtml