Well, not so much 'can of worms' perhaps as much as food for lively debate.
I think it takes.
- A photographer who can realise into an image, a recognisable and original style or 'vision' of a subject they are interested in.
- They're very clever if they can do this with a subject they are not personally interested in.
- Have mastered their craft end to end. Know how to use it to be expressive in a final image.
- As John mentions, creativity or innovation. At some point, leave behind the heros and develop a personal style. Have the confidence or fortitude or ignorance to develop the style and run with it, continuously.
- The resulting image has got to move someone who views it. The more, the better. Pluck the emotions and invoke interest/curiosity or repulsion. Or an ahah moment, that lasts.
Salgado, Erwitt, Nachtwey, Paul Caponigro are very popular and highly regarded, but very different.
- It takes relentless, passionate application to lift the quality of each high enough.
Depending on the genre the photographer works in, there'll be a different amount of each but I believe they will all be there. Or then again I might be talking a whole bunch of ...