Without experience as a public executive candidates for the Presidency need not apply.  Experience governing is critical to being succesful once in office. It is more critical than legislative experience and philosophy. Those with only legislative experience are unprepared to govern.

A likely candidate with very interesting experience is Bill Richardson (D NM). Some backround: In the news as Democratic Governors Association chair and an article about how he operates. 

In addition to Richardson's professional credentials, he would be fun to watch. What makes him an interesting human being however may also be his downfall. In this era we say we want authentic candidates, not cardboard cutouts, and then torch anyone who commits a one sentence gaffe.