For me it was their arrogant, self-worshipping attitude.
The elder that first studied with me said that the organization was just like a beast of burden for Jehovah. That sounded humble but the following years showed that the organization was no where like that.
First there was a statement I heard from the platform that went "An elderly woman once said that when the Watchtower came in the mail it was like opening a letter from Jehovah." The elder told us that was a fine attitude to have.
Then I heard the oft quoted statement: "Even if the organization is wrong how dare you correct it, you're running ahead of Jehovah."
Finally there was the comparison of a scripture that spoke about the ark of the covenant which was being transported in a cart and was in danger of falling down. Then a person went to keep the ark from falling off and was instantly killed by Jehovah for touching it. Of course the ark of the covenant represented the organization and the unlucky man represented anyone who would dare correct it.