Typically, privacy, reputation and good services like withdrawal and deposit are significant. These are the most common factors that gamblers wanted every time gambling sites are searched and reviewed.
To add some few more things that players commonly complain of if not provided:
- quick response support system
- a bonus/rakeback system
- a VIP and leveling system
Just to make sure they don't get abuse, and the gambling site should have a validated fair system and algorithm. I can recommend that you first have your own research before using a gambling platform to avoid future problems such as getting scammed.
I think the most basic and hands-down only thing to be accepted as a fair casino is the provably fair system. Without this, it is not a casino at all, if not they are only a sports book since those dont need this system. But yes for dice slots and other games where you are playing against the computer, you have to have a system such that the casino is not rigging bets. Otherwise it is a scam.