The most thing that makes me crazy about online is delay in withdrawal and kyc requirements, ie failure to verify kyc details when submitted to them and lastly delay delivery of deposit. What I don't really like is after winning and they published your winnings like not secure while dealing online so everyone casino should limit how to publish people's winning.
From it all, delay in withdrawal is the most annoying for me, i dont have a problem with kyc because I've made it a habit to apply for kyc verification immediately after signing up on any account, this is one of the reasons I don't jump from one casino to the other, this always saves me the trouble of trying to pass kyc verification when i luckily win a big amount that requires i pass kyc before i can make any withdrawals, because i perfectly understand how annoying and frustrating it can be when one start having issues passing kyc verification successfully at a very critical time, so i prepare ahead to avoid such stress.
On the issue of deposit, deposit delay can be annoying at times though, but for me, nothing annoys me more that delay in withdrawal and also delay in getting customer care response to issue that needs urgent attention.