Something new as in new kind of games
A look & feel which should look trustworthy so people will deposit money without being afraid you scam them.
Volume, people like people, so the more active the easier a visitor will stay on the site and start playing.
I still believe in ideas like arcade classic games + Bitcoin betting combination. Battlecoin.org is a demonstration of how the model can work, but the problem was these guys didn't get enough funding and the main dev can't work full time on it, but with enough polishing a website like that can be huge. I think in the future arcade games like those that allow to bet with Bitcoin will be super big, specially once we have Lightning Network for instant transactions.