For me, it's always the reputation. Everything stems from it so you can't really go wrong, they wouldn't have a good rep if they are cheating their customers or if they are being rude to them right? So I am pretty sure that reputation would be the greatest indicator that something is right with that casino that we are choosing.
^ This is not enough for me, reputation sin not enough if the services have a failure or have massive fees per payout.
There should be enough features that satisfied us as a gambler, look at this thread made by Karl
https://bitcointalk.org/index.php?topic=5382430.0, it is a voting pool about a good feature of a gambling site and I think the choices that list on the pool is quite all good. Upon picking a good gambling casino, not only 1 feature that we look at to determine the best casino, it should be more than 5 features to be considered as good.