As far as doctrine is concerned, a simple reading of the Book of Galatians convinced me that JWs didn't know the first thing about Christianity. It was painfully obvious, after reading Galatians that the Watchtower's (1) salvation by works and (2) adherence to a WT Law that has replaced the Mosaic Law are unscriptural.
The other thing that convinced me, early on, was their explanation of the Other Sheep in John 10:16. Every single other Christian religion has seen the simply truth in this verse as referring to non-Jews. WT used this single verse to create a secondary (lesser, inferior) class of Christians. Unlike all examples of Christians in Scripture, these persons do not have a desire to be with Jesus Christ, nor are they born-again and spirit-anointed.