A survey of ex-Mormons by Mister Poll asked participants to "Please indicate up to 7 main reasons why you finally decided to leave the church." The most common results, sorted by percentages, were:

General disbelief of Joseph Smith as a chosen prophet (6%)
General disbelief of the Book of Mormon (5%)
Emphasis on blind faith / Obedience to church leaders (5%)
Found more peace outside the religion (4%)
General mindset of the fanatics of the LDS religion (4%)
LDS/religious logical fallacies/manipulations (3%)
Cult-like temple ceremonies (3%)
Book of Abraham (3%)
Character of Joseph Smith (3%)
Desire to uphold personal ethics (3%)
LDS stand vs. intellectuals (2%)
Brigham Young's teachings (i.e. blood atonement, race, etc) (2%)
Polygamy (2%)
LDS vs. feminist views (2%)
Book of Mormon archaeology (2%)
LDS stand on gay issues (2%)
Fatigue/ depression (2%)
Racial issues (2%)
Unanswered prayers about LDS "truth" (2%)
General dissatisfaction with apologetic (church scholars) answers (2%). 1
Reasons Ex-Mormons give for leaving the LDS Church