Actually, you may find that you can succeed better by focussing upon particularly niches.My personal experience is that online writers can be broadly defined into two basic groups:
1/ Sunday writers, who write for themselves for the fun of it, and simply like to share what they do,
2/ Serious aspiring writers, who write with the ultimate goal of becoming published.
I don't have great experience with writers' groups - only ever joined a handful - but a particular reason I've stayed here is that there are a lot of serious aspiring writers, who can share their experiences and resources. But I don't know of many places like that.
Many of the larger writing sites seem to be very generalised, with the attitude of praising any creative effort. With leaves them tasting watery and patronising.
But serious aspiring need somewhere hard defined specifically for learning the ins and outs of submissions to the publishing world, with the aim of entering it. Perhaps there are sites like that, but I've not seen them.
So if you want to make a success of your site, my personal recommendation is that you aim specifically for serious aspiring writers, accomodating both short stories markets and methodologies, and necessary author resources and tips.
The idea of selling MSS from your place, as I said, is incredibly remote - but you could create a place where people on the verge of breaking professional can gather and learn, and thus give your site some unique kudos as they succeed.
[This message has been edited by Chronicles_of_Empire (edited July 03, 2002).]