I think the most important is trust, now a days the well established sites are trusted and very successful while if a new site comes along people dont really trust it and only gamble in small amounts to try out and see if it can be trusted. Secondly i think marketing and promoting a site is a necessary thing, if the site provides time to time promotions like free bets or giveaways people are more likely gamble on it. 

I personally prefer both, some games like dice can only be played with depositing some funds and i like playing dice from time to time. But some direct blockchain games like luckybit is really good to gamble directly with no hassle of depositing and withdrawing.