"Standards", if existing at all, will vary in each dojo and also with each individual involved. The experiences I've had with testing leads me to believe that while it's true that some degree of technical proficiency must be maintained, there really is no "baseline" to standardize against. This makes sense when you consider that we are all individuals and will interpret what we see and feel a little different.
Again, I can only relate to what I've experienced and my views certainly do not represent anyone but myself, but it appears to me that it's really about the persons commitment to their training, to the dojo (and fellow practitioners within), and whether the grading panel feels that the person "testing" is really doing their best. After all, you can only offer your best efforts. To standardize is to compare, and just who is it you're willing to be compared too?
Other than the names and order of techniques to be performed, I don't believe there can ever be any "standards" in Aikido and perhaps, that may indeed be one of its greatest strengths….
On a personal note, I'd like to see the ranking system either done away with completely, or at least receded to it's correct meaning, that is, a personal understanding between a practitioner and their sensei(s).