I will say hosting, a nice, easy hosting, where you can plug and play, and where the URL's are designed to be easy to find. wordpress has this great, I went a similar route with DF by making folders dreamfantastic.com/yourserialname/ , the way some others do it, like JP, seems a bit... obtuse.
Community. I want a group of existing readers and writers to share with and communicate with. obviously this can start small, but should have room for growth, which leads to
COMMUNICATION! I want a single chat thread for a story as a whole, say on the title page, a chat thread (and NOT a fucking sidebar. FULL WIDTH, under the story. Sidebar comments makes the baby Jeebus cry. ) for each chapter, and a group message board where each story has a folder for longer ongoing communications/ wild mass guess/ other types of communication threads that go beyond single chapter comments, as well as sections for talking about OTHER things, to create that sense of community, and some private boards for the writers to discuss the craft and make their own sub community. The boards over at GiantITP.com (order of teh stick) are WONDERFUL. Not everyone is going to use all those methods, but different people prefer certain ones. GIVE THEM THAT CHOICE!
And... NOTIFICATIONS! I want the option for indivudual or daily emails with updated conversation information and friends/reviews. I want to log in to the central hub that all the stories are under, and get a nice number telling me how many people have replied to my own comments, with links to take me straight to their replies. I want PMs GOSHDARNIT! I want RSS feeds for each individual story I read, and a nice easy button to get it.
Reviews. I want a review system that makes sense, lets me get an idea of the general feel for a story (dont give me an average, show me the percentage of awesome and shit, like the google play store does) and is self policicing with highly regarded reviews showing first. I want an obvious place to see reviews of a story without haveing to READ the story first.
On the comments others made, Billy, i tried to run Dreamfantastic as an editor. I made story suggestions, fixed formating and typo issues, made suggestions on readability of the sites. It didnt go over well. Some people might LIKE that, but i really think unless you have someoen with known editor chops running the site as basically a publishing label, which gets into rights issues, its not going to be as popular as you think. (im always willing to prove myself wrong!)