It's very close between fast payments and trust/reliability. I think in practice, they are often related.
Fast payments:
I had one casino playing games, delaying payment (barely $600) for weeks. That was a thorn in my mind as I don't like having unfinished stuff on my plate. My main casino pays me out thousands within minutes. No hassle.
Trust:
A casino that honors the game, doesn't try to screw you over w/ vague, open to interpretation rules, is a gem. There are casinos that will steal your winnings on a technicality... on a previous deposit which you bet down to zero.
Good bonuses & rewards:
Playing slots w/o some sort of extra incentive or goal (e.g. bonus wagering, reel race type tournaments) feels aimless and endless. Although, the lovely forum contests held by CasinoGuru also serve this purpose.
Game selection:
Having lots of games and providers is nice to have. It's annoying having to hunt down games, depositing at several different casinos just to play a specific game or three.
Helpful Customer Support:
I try not to interact with customer support: they're almost always bad, but snappy, helpful support is another nice to have. I did get Play'n Go slot RTP made public (they're usually hidden) at one casino by asking the customer support. That's about the extent of my positive interaction with online casino customer support...
Community/social features:
Easily the least important on the list. For me, gambling is a solo endeavor and socializing can come afterwards, on forums, once the gambling is finished. I personally don't even want to see any social features during a gambling session.
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