Here is my own list below.
1. Provably fair
2. Instant withdrawal
3. No KYC verification
4. Has free faucet for testing games.
5. or have free good bonuses
6. Low House edge
7. Casino with best odds
8. Support more altcoin.
9. Fast website
10. No bugs
This is what other gamblers looking for.
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This is the bare minimum that you should go for. You're going to be going against other casinos that have the same, so you have to differentiate yourself further if you really want "make it." Look at what other casinos are doing poorly and try to see how you can make a difference