The fact, that they broke away from strange and unbiblical (my opinion, no debate required ;-)
teachings like hellfire, trinity, idol worship etc...and then, once you trusted their reasoning, they introduce through the back-door things like:
manmade rules (about holidays, blood, JC etc)
GB worship
stretching of scriptures ("slave class", little flock, head covering many many more)
guilt and fear
and: You swallow it, because they taught such precious truths (see above), you turn off your brain, once you trust them.
This tactic is visible in the main study aids:
first the "bible teach" book, containing mostly reasonings which I still believe to a large part , like introducing God, Jesus, Gods purpose, resurrection hope etc...all nice, and has at least some substance...(most people dont care much about the 7 times anyway, as long as they find confoirmation, that our days are the last days....)
and after you learned this, they come with the new "rule" book, "Gods love"....I wonder how many would continue study, if you started with the Gods love book first, when they still have their brains turned on...