Primarily trust. Trust that the facilitator of the games will be fair and transparent about the process. Also, it is important that you will get a payout and therefore you must have trust in the payment facilitators. Payment facilitators that pay out in real money, usually will also to some extent protect you from fraud or shady dealings. We explain this in more details in our individual country guides.
Secondary to trust is the quality of games provided. The quality of games determines your enjoyment and experience at the casino. This includes speed of game executions, down time of server if any and diversity of games. This is something we discuss in greater detail in our online specific guides.