I would say make the house edge small, small enough to gain a profit but not drive people away, A week or so ago I saw a scam-dice site that had a house edge of 75%, and only 10 people were using it--and the 10 people that were using it lost all of their btc they had deposited within about 10 minutes of betting. Now look at Primedice, house edge of 1%, and thousands of people are using it every day.
Another thing is make the GUI "pretty" --- but efficient, and not have input/output lag.
Example: 999dice, if you try to bet, at least for me (and I have a big gaming PC and great internet) it takes about 5-10 seconds for the bet to go through, but Primedice it takes about 1 second or lower.

I hope this helps a bit