People always ask me about the criteria that I use to determine rank. This question comes with a large set of subjective answers that varies from teacher to teacher. The knowledge or skills set is different between kyu ranks or mudansha and black belts or yudansha.
For me, mudansha students are evaluated on what I like to call their Aikido conditioning. Their Aikido conditioning refers to things the student needs to get into shape such as their ukemi skills, rolling ability, and basic technique differentiation as both uke and nage. At the mudansha level, technical nuance isn't that important as long as they can perform the movement in a fairly competent manner and perform the right ukemi with each technique.
For yudansha at each level, the precision becomes greater and the student will be required to demonstrate everything they have learned up to that point. At shodan, the student will need to show general knowledge of the technical form and display the Aikido conditioning they have built up. Since shodan is the proverbial beginning, technical differentiation is more important than technical nuance while serving as both the nage and the uke. Being able to do the techniques being called out and the proper ukemi is enough to pass the shodan exam, but not enough for subsequent black belts. At each level of black belt, the student is evaluated on their technical precision, grasp of timing and spacing, breathe control, and mental awareness or basically where they are supposed to be and what they are supposed to be doing.
The last and most important at all levels is the candidate's character. A student with no athletic gifts and a great attitude will go farther than an athletically gifted student with a bad attitude and I would promote the former a 100 times over the latter. I personally will not promote anyone with a bad attitude or corrupt character.