I can tell you what I felt for the last 10-12 years I was in, and I suspect that it's fairly common.
I thought the Borg was right in the broad strokes such as the millennial reign of Christ and the future paradise earth, but it was painfully obvious to me that they were wrong about something in the time frame. The 1914 generation was dead and the 1995 change to the meaning of "generation" was unsatisfying. It was obviously wrong, and I think it was obviously wrong to pretty much everyone who noticed it. Also, I was never down with shunning. Because they were so obviously wrong about their generation doctrine I think I was primed to accept that they could be wrong about other things, too.
It seems to me that there is a percentage of hard core, non-thinking JWs who believe everything, even when it makes no sense at all. There is a percentage of free-thinking JWs who hardly believe any of it but remain in because it's their identity/lifestyle/family etc. And the majority of JWs believe most of it but have certain issues they disagree with and keep quiet about - these can run the gamut from doctrinal to procedural issues and from one or two things to dozens.
So for the latter two groups, why don't they leave? Cult Mind Control.