This is an easy one, what really drives me crazy when it comes to online casinos is slow customer support, this is because it does not matter how efficient the casino in which you are playing could be or how good their programmers are, at some point you are going to find an issue with the casino and you will need to contact customer support, and in those cases you need to wait for days just to get an automated response which does not solve your issue, and this is quite exasperating as it is obvious casinos are making a lot of money and yet they do not make the effort to have good customer support.