Some of the things that had the biggest impact on me then, wouldn't really be so now... because I've actually come to a place where I no longer believe in the bible. Back then, though... the biggest things were-
1-The fact that we were only allowed to use the NWT, without any secular affirmation of accuracy in translation. The truth can stand up to evaluation, period.
2-When you look at the organization on a whole, with the many rules and detailed limitations... Counting hours, ranking systems, rules, etc. Does that seem more like Jesus or the pharisees? Which should we want to be like? (honestly this can go for a whole lot of sects)
3-Prior to 1975 all of their literature spoke of itself as a prophet, the very "mouthpiece" of god. 1976 that changed to claiming they were not prophets and have at times had the wrong expectations. They were imperfect people. I verified this with my Watchtower CD**. When I was still a believer in the divinity of the bible that information balanced with Deut. 18:22 makes it clear that once someone has presumed to speak for God and it has not come true, they are are no prophet of God and you should have no fear of what they profess. It was really a big chunk of my feeling a relief and a lack of guilt for my choice to leave.
4-Colossians 2:16,17 and Romans 14:5,6 clearly state that people should continue to honor the days they felt compelled to without judgment. Even the days that revolve around a new moon phase, which is clearly pagan in origin. John 10:22 shows Jesus going to the Temple during the feast of the dedication. (Not exactly being "no part of this world" by JW standards.) Yet JW's would be DF'd and shunned for choosing to honor any days they felt compelled to... in opposition to the first two scriptures given.
**Watchtower 1959 January 15 pp.39-41, Watchtower 1964 June 15 pp.365-6, Watchtower 1972 April 1 p.197, Watchtower 1976 May 15 p.297, Reasoning From the Scriptures (1985, 1989) p.136