Not sure how much weight it carries with some operators these days but actually passing a Command Course identified one of the differences between Captain and First Officer.
Basic requirements for selection were, qualification, (licence and type rating), experience, (at least the minimum number of hours required for a Command on appropriate types and in appropriate areas of operation), and suitability, (considered by the company to be ready for promotion based on seniority, attitude and demonstrated competence).
I think that to suggest that the only difference between Captain and Co-Pilot is seniority is to gloss over some rather significant hurdles that have been the downfall of quite a few aspirants in the past. Anthony Carn and Max Angle have, I think, nailed it in one.