Since it's impossible to find complete harmony within the bible itself, you're always going to find it difficult to completely support almost any theological position. However, I find that JWs generally have only a few scriptures to support major doctrines, sometimes just one, and scriptures often taken out of context, whereas mainstream Christianity will often have a lot more support for their doctrine.
'Some have stated that the JW's go too far with their doctrine, but that much of the basic beliefs of the religion are correct.'
I can't think of any basics they have right. They mess around with the really, really obvious basics. Like Jesus' command to 'love your neighbour' which is possibly the most basic command in the bible, yet their official position is that only JWs are your neighbours, which is clearly contrary to Jesus' parable about the Good Samaritan, showing what neighbourly love meant. They might say they show love for neighbours by preaching, but I didn't see the Good Samaritan ever preach to the Jew he helped out. Of course, JWs will confuse this by claiming basics are things like 'No Trinity, No hellfire, No immortality of the soul' but if there's no exact harmony on these points in the bible, are these really the basics? Or are the basics Jesus' message of love which the WTS constantly misses the point on, and constantly adds conditions to?