I think it would probably depend on how well the 1st kyu looks/feels as both nage and uke. Physically and visually aesthetically pleasing to both the partners, the teachers, and those who are watching. This is all IMHO of course. I know different people have different ways of judging, but I think that which is beautiful can be observed and appreciated across people.
I guess a bigger question would be how does a teacher judge if he/she would like to use someone often as an uke for demonstration? There has to be something about them they enjoyed, unless they're correcting them.
Although... how exactly does one judge whether or not a technique is done even correctly.
Now my brain hurts.
Thank you. I understand what you mean.