This week, the Diocese of Springfield (in Illinois) released the results of surveys they did of Catholics and ex-Catholics in their area. They interviewed about 1400 people, and the work was done by a third party to prevent bias or a skewing of statistics.
The #1 reason people left the Catholic Church was because they disagreed with what the church teaches. This was mentioned 47% of the time. I thought it was interesting how this coincided with education levels. If you went to grad school, you are more likely (on some topics, twice as likely) to disagree with the Catholic Church.
The issues people disagree with most are birth control, the fact that priests can't get married, divorce and remarriage, fertility treatments and homosexuality, in that order. I am surprised that fertility treatments were mentioned over half of the time, while abortion was not.
The #2 reason people left the Catholic Church was a lack of connection to the Church. This was mentioned 38% of the time. To put it simplistically, people got bored and left. A majority mentioned that their spiritual needs weren't being fulfilled.
Reason #3 and #4 were about tied, around 15%. These are church scandals and a lack of Christian atmosphere. People over 50 hate church scandals more than people under 50, for whatever reason. As for lack of Christian values, the main reason seems to be that the priests / other Catholics are jerks, so people leave.
The second survey was for practicing Catholics, and they indicate a lot of the same ideas (they don't like it when priests are jerks, they prefer to feel a sense of community, they dislike the church's view on birth control more than anything else, etc.). The improvements people recommended most were changing the mass and/or the priests.
As a Catholic, I wonder what I can do to help, besides for "stop being so anti-social when I'm at church".