Hi Airduster, welcome to the board! In answer to your question, for me it was the following:
1) The lack of love - the climate of condemnation, judging, guilt trips and the message - you will die, die, die if you don't do xy&z. I was afraid of Jehovah, I didn't love him and I never thought he loved me, I just wasn't good enough, and where was Jesus in all this? - now I know my God and my Lord, I am approved, loved and they are pleased with me, I never knew that as a JW. Also on the surface there is warmth and love but the second you think, act, speak or do anything contrary to their belief, make no mistake, you will be cut off, that happened to my mom and It hurt me to see the way she was treated like dirt by family and 'The friends'. And years later, it happened to me.
2) I was miserable! - For being part of the happiest people on earth, I was so sad, trying hard to do everything they asked and it was never enough. I saw the faith and joy of Christians and I began to wonder, how was that possible without the 'truth'?
3) My doubts - I just woke up one morning and had to face them. At the time, I was a reg.pioneer and scared to death. I never read anything 'apostate' or spoke to a former JW. I just knew in my heart and soul that I didn't believe that God would wipe out the majority of the earth's population just so that I and a few others could live in paradise. I hated the Armageddon pictures. I hated the gleeful way 'The friends' would watch the news and hope for the end. I thought, I'm not hoping for my non-JW family to die, I don't want the 'worldly' kids next door to die. It took me 5 years to finally leave because I didn't want to face the fact that it never was, never will be 'The truth'.
I wish you happiness and peace, God Bless - V Sky