They appeared to understand the Bible, including parts of it I knew nothing about.
The other factor was that no non-Witnesses I asked took the time to defend what they believed from the Bible, other than citing 1-2 verses.
They would tell me that the WTS took passages out of context but didn't show specifically how or what the correct context was.
In retrospect, I think they didn't really understand what the Watchtower was saying about various doctrines and so didn't take many of their teachings seriously.
It was as if they considered WT doctrines to be obvious nonsense that didn't require refuting.
That approach has probably contributed to a lot of people getting sucked into the borg.
One of the things that led me away from the WT was a set of tapes by Walter Martin and Bill Cetnar.
They did take the WT arguments seriously and took the time to refute them one by one.