Here's another taboo, the unlawfully authorization of customer's private information to 3rd parties and illegal selling of those private information like KYC (name, home address, picture, IP address etc) on the darkweb. Exchanges engage in this act and there isn't much we can do about it.  Also, excessive withdrawal fee. Hitbtc exchanges is a very popular example as the exchange is guilty of excessive withdraw fees, I won't advise any one to use them.