The core question here should be, why do we even have separate CS and CS Theory sites?
And the most convenient answers to this can be found by reading this thread on the CSTheory meta.
If you have time, please read through it before continuing here. If you're still stuck in line at the DMV after that, read this. But if you have time to read nothing else, please at least read this answer from Jukka Suomela:
Not everyone agrees with the premise that StackOverflow is exemplary and that all other sides should follow its model. I find its huge volume of traffic exhausting and daunting. It is too fast-paced for my taste. Questions are asked, very quickly answered, and then forgotten. It might work fairly well for technical programming-related questions, but it is not necessarily a model that I would like to try with theoretical research problems. I think that reasonably low volume is an important feature of this site.
While perhaps not stated in such plain language elsewhere, this seems to be the underlying theme of most answers there, and it represents a division of philosophy that arose in the SE 1.0 era (beginning with Stack Overflow) and has now been replicated and expanded on the SE 2.0 sites. There are a number of ways to describe this division, but for the purpose of this discussion I'm going to use a question...
Is a lack of existing knowledge of the field an insurmountable barrier to entry?
Stack Overflow espouses an open-arms approach: show up, ask your question, and as long as it's about programming - an extremely broad field - you'll get an answer. Doesn't matter if it's answered on page one, chapter one of Learn 2 Program in 10 Hours - they won't turn you away.
Theoretical Computer Science takes the opposite approach. Your question must be related to theoretical computer science - an already very narrow field. But it must also be a research-level question. From the FAQ:
Although there is no black-and-white distinction between research-level questions and non-research-level questions, questions are considered to be "research-level" roughly when they can be discussed between two professors or between two graduate students working on Ph.D.'s, but not usually between a professor and a typical undergraduate student. It does not include questions at the level of difficulty of typical undergraduate course/textbook homework/exercise.
The big tent
In practice, there are questions that are considered "too basic" for Stack Overflow. But this inclusionist philosophy has had a massive influence on its reach and scope: by demanding little-to-no background from askers in any topic, SO has been able to serve a staggeringly broad base of users from different backgrounds, many of them experts in their own right but in fields that have only a tangential connection to programming.
But even more importantly, this broad scope encourages the collaborative construction of a library of knowledge useful to many who may never actively participate on the site. This was not accidental. And the success of this philosophy in achieving this goal is reflected in the numbers: 96% of visitors to the site do so because their question shows up in a Google (or other general-purpose search engine) search result. Network-wide, that number is about 87%. The vast, vast majority of people learning from the answers on these sites will never need to even sign in.
Oh... And on CSTheory? That number hovers between 40% and 50%.
The mailing list
This brings me back to Jukka's answer (quoted in part above). Even though the Stack Exchange software seems to be working fairly well for that community, this is mostly accidental - it was never designed for small, low-traffic, mostly self-contained groups of people. You can actually license the system for use on private, internal sites, but very, very few groups ever do this: not many internal organizations reach the scale necessary for it to actually work.
Indeed, CSTheory is more akin to a large mailing list or message board than it is to the sort of Q&A repository Stack Exchange was designed for. That it works at all, and has managed to establish and sustain a core group of users, is actually quite fascinating. I feel strongly that there are lessons we can learn from it and apply to other topics within our network.
That said, I must strongly caution you against attempting to replicate that philosophy here.
We're not in the mailing list business
Please let me be frank with you: this network - which is to say, the people who comprise it, not the organization funding it - cannot support a dozen sites like CSTheory, now or in the near future. And I have no desire to shut this site down, followed one after another by each of the dozen or so related proposals for niche sites. But if each one insists on trying to follow this mailing-list pattern, that is exactly what will happen. That's not a threat - it's a prediction.
So to finally answer the question, this is what you can do to make your community different from that on CSTheory:
Computer science is a fairly large field, and folks approach it from many directions. Some make it their field of study; others make it their livelihood. Many will encounter it as part of their work or study in some other field. Don't turn them away.
Welcome closely-related topics
Don't worry too much if a particular question seems like it's focused more on mathematics, or software engineering, or statistics. If it can be asked and answered from a CS perspective, then edit to make that clear... and then answer it. I would much rather see this site encompass topics like artificial intelligence or even computational linguistics than try to spin up separate sites. Be very careful when defining your scope to be as inclusive as possible without completely abandoning CS as the focus.
Strive for accessible language in questions and answers
Every question need not devolve into a beginner's tutorial on the basic concepts. But questions and answers will be far more useful as a resource for others when they're written (or edited) with that in mind. Avoid ambiguity, embrace detail, and welcome questions from those unfamiliar with your particular area of interest as an opportunity.
Be patient with students
I have a sneaking suspicion that the primary audience for this site - initially at least - will be students, from many different backgrounds and at many different points in their education. This is a wonderful opportunity, but it can also be a burden: students are often poor at asking questions, may lack sufficient background to even know quite what they're asking, and can be tempted to misuse the site as a resource for cheating rather than learning.
I can't promise a happy outcome, even if you follow these guidelines. As some of you may know, I was quite reluctant to see this site launch in the first place, and I still harbor grave doubts as to whether it can be made to work. But after much discussion, in public and internally, we decided to give it a chance.
All I can ask is that you do the same...