Not paying out winnings
Of course this is the quickest way for an online casino to find itself on our blacklist. It's the cardinal sin; the ultimate betrayal of a players trust.
Let's face it - while people play casino games for enjoyment, for fun, for the thrill of it, at the end of the day they're trying to win money. Goodness knows with the house edge and the odds that we face, it's difficult enough as it is. So when a player does win and the casino refuses to pay out, or finds all kinds of obfuscating reasons to delay paying out for as long as possible, that's completely unacceptable.
Of course there are variations in the payout procedures of the different online casinos. Some of them provide immediate withdrawals. Others require verification steps, like proof of address, before they will pay out. Some of them pay out over a 30-day period. And if a player hits a particularly huge progressive jackpot, some will pay this in instalments. All of these are acceptable parts of the payout process. We don't blacklist casinos for these things.
The ones that are blacklisted are those who simply never pay, using delaying tactics to keep telling you that "the cheque is in the mail" - when ultimately it never arrives. Or they find all kinds of minutiae in their terms and conditions to prevent having to pay out. Of course if those deposit terms or bonus conditions are clearly stated up front, that's another matter. But some casinos go out of their way to hide onerous payout requirements to deliberately create a situation where players are enticed to play, but never have the right grounds to be eligible for payouts.