I figure a track record in high elective office is essential. Ideally the candidate to have served in both a legislative and executive capacity, but since one is usually all that's on offer, the latter is best since the Presidency is itself an executive position. The candidate should have dealt with important issues rather than merely talked about them.
It doesn't mean that someone without those qualifications might not be brilliantly successful, but the electorate shouldn't be asked to take a chance on an unknown quantity in the world's most critical job.