A couple of other ideas:
I do know that 'sometimes' I am in a rush to send back disks, because I didn't get the ones I was in the mood for, which points to the fact that we can't set a priority for our disks. 2 in the ASAP list is a good start, but the fact that it seems completely random from the main list is confusing. I may actually rent less disks if I got the ones I wanted in a timely manner.
I understand why they don't have a ranked, ordered, system, because people would get confused when they got number 30 on their list. (People don't obviously think of all of the factors involved.)
If they had a preference or something that stated a general 'priority' setting, it may help to clear up peoples lists. For instance, "oldest selections first" (this would be the order you placed them in the list) or "newest movies first", which would mean it would ignore the order you put them in your list and ship them newest to oldest. How about ""Highest IMDB rating"?
Another idea would be to specify a "mood". This way, you could select "Comedy", "Drama", "Newer Release", "Classic" or "Independant" as a preference. The movies are already categorized in a similar manner.
If you try to compare this to a real life experience, what is the first question the video store employee asks you for when you ask for a suggestion? They ask you what you are in the mood for, and then make a few recommendations.
I think if they tried more options for getting people to stay away from focusing on new releases, they would have happier customers and maybe less anxious people. Most new release movies aren't necessarily the best movies on people's lists, its just that they are the ones people are "anxious" to see.
How many times have you received a movie you weren't really in the mood for, and anxiously sent it back so you could get something better? This could be increasing the volume turnaround for Zip. (just a theory).