The national Danish Aikikai has issued a curriculum for kyu gradings and 1. to 3. dan gradings. It describes which tecniques the student should show and there is a table describing which abilities the student should be able to show at each level (eg : 'Knows the technique', 'Moves in a confident way', 'Maintains proper Maai' etc.). This is a guide though, and it is common for some dojo's to ajust the curriculum a bit.
Anyway my sensei once said that with regards to kyu gradings he could pretty much tell what grade a student should have just by the way he walked on to the mat.
What could influence the result of a grading would also be the understanding between sensei and student - as Mongo pointed out above. We never have a grading without a following oral evaluation, and that's really where the meat is, since that's when you get an idear of how your sensei think you have evolved and what you should work on.
First and foremost it's about training.