What's this site's value proposition?
Looking at this some more here are some ideas...
1) There is a very small community of people who do algorithmic trading, this is probably your core audience. Unfortunetly we're scattered all over at places like http://www.wilmott.com/index.cfm and nuclear phynance. What you do to get people like us probably dictates how successful your site becomes
2) There is a much bigger set of people who like the idea of maybe one day doing algorithmic trading or like to keep up on it once in a while. These people will bring eyeballs but probably add very little of value to the discussion.
Not because they are less worthy but because of the same reason the average person can't contribute to mathoverflow.com, you need to have your head in this space almost full time to contribute.
So I think the question you need to answer is:
Why is a semi pro trader( someone who trades up to 2 million of their own money), taking 30 minutes out of their day to read your site?
What value do you bring over already established discussion forums?