The most important thing is always the marketing.

I became friends with one of the biggest casino operators of crypt at one time (they are not longer famous but they were probably the most wagered at one point) and we talked about business all the time, he liked talking to me because I always said what could be improved and he cared for such things. He built the whole website for 50 thousand dollars, now that is not a small amount but that is not that big neither, after all who doesn't spend either 50 thousand dollars on building a casino or at least 50 thousand dollars worth of effort. He said that was the simplest thing because he was capable of affording good developers and give them salary and so forth which made his website good, however even with a good website the marketing was the biggest challenge.

Dude had to spend 10 thousand dollars per week just on bitcointalk with ads and signature campaigns to get more famous not talking about all other places and ads, so it took him about half a million dollars spent until he was making more money than he was spending.