Looking at the codex, the Contributor role seems a good fit. They can create (edit/delete) posts but not publish. Maybe something that allows anyone to have contributor access without an actual user account.
Maybe the ability to use an existing account (OpenID, Facebook, Twitter) to create a contributor account on the blog would be a backup plan. I'd just like to avoid the extra user/pass account creation as much as possible. I feel that submissions will be more likely this way.
I may just need to go with the built-in solution. Set 'New User Default Role' to Contributor in Dashboard/Settings. Maybe adding something like the following plugin to enhance registration.