My experience is very similar to Magnum's experience. It was a combination of many things. Crazy as this sounds now....I actually wondered why we went door to door witnessing when it was obviously so ineffective. If this was the most important work on earth, why weren't we putting some more effort and instead just doing this busywork?
I'd always had gnawing doubts about JW history but when we studied the Revelation and Daniel books....that was just insane - I didn't understand/believe any of it, yet I still continued because it was all I knew and it was familiar and comfortable.
But I guess what really cinched the deal was realizing that people who were non jw's were more stable mentally and emotionally and had fewer issues than those inside. I'd always been taught we were so "special" and then when talking to non JW's I realized we were actually a bunch of wackos. I came to realize this religion attracted people who were coming from a damaged place or were easily manipulated because of their lack of education and critical thinking skills. And the amount of unloving/crazy behavior in the hall. Outside the hall, that behavior made no sense. If you had good intentions and actually wanted to help someone, there were all these obstacles. Would it stumble someone? What did the literature say on the matter? What does elder so and so say about it? By the time you got the answers to all that, you didn't even want to help the person anymore.