Assumption: we want as many professionals on the site asking as many "long-tail" expert questions as possible. From Joel Spolsky's post re: firearm SE proposal:
Excellent suggestion. One thing to think about is this: is there a subgroup of firearm owners that are more than just dilettantes? Think about why MathOverflow is so much better than Yahoo! Math Answers... it's because it attracts professional mathematicians. Can you narrow the target audience from "people interested into firearms" to something like "professional firearm dealers"? Or "military firearm maintenance officers?"
You'll still get all the people interested in firearms, but because of the heavy presence of professionals, you'll get much higher level conversation. Rather than "What are the requirements to obtain a concealed carry permit in [state]", which already has an excellent Wikipedia article, you'll get questions like, "How do I authenticate a genuine 1911 Pacific Theater commemorative Colt?" which doesn't.
The power of the Stack Exchange platform is detailed, expert answers to extremely rare, "long-tail," highly technical questions. To get expert answers, you need experts. To attract experts, you need a site where people are asking very interesting and hard questions, not the basic questions, so that it's clear that this is a PRO site, not a consumer/enthusiast site.... and remember, the pro sites WILL attract the enthusiasts, but not the other way around.
I think it's going to be difficult to get the kind of long-tail questions that really drive professionals to the site. Most of the major players in hardware and software put developers under pretty stringent NDAs. Posting anything about the Wii/PS3/Xbox/beta iPhone SDKs isn't really something people can do. That's probably true for most game engine licenses as well. And there's the idea that a lot of proprietary tech is considered a valuable trade secret.
So a lot of the questions on the site now are high level subjective design questions. The engines and platforms that aren't under NDAs are generally considered indie/enthusiast (i.e. XNA, Unity, UDK). Non-domain-specific programming specific questions usually are the domain of SO.
I don't really have a specific question to ask. More like a call to arms on suggestions on what we can provide as starting points for good questions to bring people here.