For me it was about 15 year's worth of straws that eventually broke the camel's back. Early on I asked questions about lots of these but was just never satisfied with the answer I got (which only amplified my doubts). Later on I just stopped asking questions. Then the overlapping generation nonsense combined with a mention of 1975 not necessarily being all private speculation by the R/F led me to ask "how are we different than Harold Camping" and that's what finally pushed me over the edge.
Some of the key factors from memory, roughly in order:
- If JWism is the only true religion, why does it seem that most converts are a little nutty an no one with any intelligence seems to be brought in? (this is a sweeping generalization based on the congregation I grew up in, I mean no offense to anyone who converted as an adult.)
- Why does it seem that the rules around dating appear designed to get kids married off in their late teens, in spite of all empirical evidence demonstrating that that is a horrible idea?
- Evolution seemed to be a much more elegant explanation to speciation than "god did it."
- Earthquakes aren't actually increasing in spite of the WTS's claims.
- Disfellowshipping sure seemed like something you'd institute if you were trying to keep people in a cult.
- The prohibition on higher education also seemed like a good idea if you were running a cult.
- The prohibition on outside research seemed like something a cult might do.
- Violence condoned by the horrible god of the OT
- The bible's claims about the last days are all so vague that just about any generation could (and has) applied them to the time in which they live.
- If WWIII broke out in 1975, would that be a pivotal year in JW cronology (similar to 1914)?
Once it got to that point I started looking around online. The 607 thing didn't phase me too much (I wanted black and white proof one way or the other, and since 607 didn't happen in my lifetime I couldn't verify it either way) but when I found the flat-out lies in the creation book (Thanks jwfacts!) a switch flipped in my brain and I was mentally out.
So, for me, I would say that it was 100% doctrine that led me out of the cult. I decided long ago that if it was the truth then I would commit to it no matter what it entailed. I never felt mistreated while I was in, my only source of discomfort with the cult was that there was just too much that didn't make sense and couldn't be reconciled.