I think that what Peter meant, was that these categories are often perceived as unreachable or too hard, when in fact they're a good option for high achievers, especially those who are proactive, motivated, resourceful, and detail-oriented.
You're right, it WOULD be helpful to see a yardstick, and some examples on a continuum of high-medium-low qualifications.
Here are a few news stories I've collected about real people getting their O-1 visas, and what the process was like. They give you at least a few snapshots of successful O-1 applicants.
I also wrote a blog post that gives an overview of the evidence you need.