If I ran an OLD site, there are certain questions people would have to answer. I know a lot of people lie, but I still think these would be good questions.
1) In addition to the married/single/divorced/widowed question, I would ask, "If divorced, how many times?" I think most people don't mind a person who's been divorced once, but someone who's had three turns on the marriage-go-round has a few red flags.
2) Are you OK with having dating someone who already has kids? (Got the idea for this one from a previous post by...musiclover, I think.)
3) What are your three biggest faults/foibles?
I've also thought it would be helpful to ask people what their previous spouses or girl/boyfriends thought was their worst qualities, but while it might be helpful in some ways, I think that's too harsh a question to ask someone.
Any others ideas? What would you make people answer in order to join your site?