Most lawyers, or at least most good lawyers, in my experience, are interested in the law, and like discussing it with other lawyers. The same goes for discussions with non-lawyers, under some circumstances.
The things that annoy lawyers about discussions with non-lawyers are more or less the things I've heard doctors complain about. Doctors don't want to talk to people who are just pumping them for free advice, and they don't want to talk to people who think they know better than all of modern medicine because they saw Jenny McCarthy on a talk show once.
I don't know about attracting lawyers, but the best way, in my opinion, to keep from driving lawyers away from the site is to avoid these two situations:
Lawyers won't post if they think they're going to face professional discipline for posting. The more questions that deal with legal theory and legal generalities, and the fewer that are outright, or thinly disguised, requests for advice for a poster's own specific legal difficulties, the more likely a lawyer is going to want to answer it (for free).
Lawyers have to deal, all too often, with the nutty fringe-on-the-flag type of legal "truther," who wants to explain that he's not subject to the court's jurisdiction because the summons used capital letters. If the questioners, voters, and commenters are hostile to real-life answers and the legal system, lawyers will take a pass.
These are things that will depend partly on policy, and partly on the user community.
It might not hurt to prime the pump by promoting the site (now that it's public) in places where lawyers and law students are likely to see it--Above the Law, for instance. A critical mass of people who know what they're talking about is probably the best way to avoid this type of problem.