What actually do you want to hear, we pretty much talk about all their shady activities, right from their outrageous listing fees to manipulation of trading volumes, decieving traders with fake buy and sell orders and sometimes inside trading.
Nevertheless, if I was to pick one I'll consider the inside job of hacking themselves to run away with traders fund as the biggest taboo. People don't talk much about this but after reading how some hack of exchanges occurs, I begin doubting, if the hack wasn't an inside job.