Doctrine was my initial reason. My congregation was too "nice" for me to see the real problems. But I was curious, being raised in it. And once I learned about internet newsgroups in college I began looking deeply into things, much more than the average Dub.
Eventually the first teaching I ended up disagreeing with was the "everyone (meaning 99% of the world) who dies in Armageddon won't ever be resurrected". It didn't make sense if nearly everyone before was resurrected. That led me to reading the Franz books, etc.
LucidSky