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.
i agree with the above-mentioned list. but how about the gambling licenses? i think if he is seriously building his online gambling business, he needs to consider the legalities of his site.
having a license means the site has long-term goals to stay in the business.
the list above can be accomplished by a good developer but he should be considering the business side also...