I think it is significant that I started having major doubts as a baptized JW at about age 30 (the same age that Jesus was allegedly baptized and began his ministry). People shouldn't be allowed to officially join a religion (especially a cult or cult-like one) prior to age 21, especially prior to age 18. In the USA people can't enter a legally binding contract until age 18, so they also shouldn't be able to officially join a religion prior to age 18 and people shouldn't be able to baptize them as babies (nor at any other time against or without their consent).
Major things that caused to seriously doubt the religion were the following (not necessarily in temporal order and not necessarily in the order of importance).
- The Revelation book claiming that the resolutions in the 1920s-1930s (or so) were the trumpet blast of angels (or Earthly fulfillments of such)
- Seeing increased evidence of biological evolution
- Seeing doctrinal changes by the WT during the years I was a baptized JW and learning more about doctrinal changes that took place before I was even born.
- Learning more about the history of the WT religion by studying the Proclaimers book which came out around the year 1995 and learning more about failed prediction of the WT.
- The WT specifically changing their teachings of: when the great crowd comes into existence, the generation of 1914, Christ coming in his glory and when the sheep and goats get separated, blood fractions, WT's former UN NGO status, etc.
- Learning things from the internet about the WT, including about Beth Sarim
- Noticing that the Great Tribulation hasn't started yet and that 2 decades (in 1995) had passed since the year 1975; changes regarding the length of the creative days
- Noticing the flawed process of appointing servants to the congregation, when according to the WT the appointments are made by Jehovah and that the elders merely recognize Jehovah's choice for the appointments.
- Noticing to prayer to Jehovah for help doesn't work, even praying for help to conquer a habit which according to the WT is disapproved of by Jehovah God.
- Learning from the internet that biblical scholars and archaeologists are overwhelmingly in consensus that Jerusalem was destroyed in 586 BCE or 587 BCE instead of 607 BCE.
- Seeing evidence in the WT's publications, including the way to quote non-WT publications, that show the WT is dishonest in many things they say.
There are probably more things I could add to this list, but I have to do other things now (nearly time to start getting ready to go to work) and thus I won't add the other things (if any) till later.