I'd like to see an objective, relative measure of how hard it is to memorize something. Let me explain.
After 25 years performing music, and notably 10 years performing orchestral chorus works from memory with the Boston Symphony Orchestra, I have had a lot of time to think about the process of memorization.
These thoughts are centered around memorizing music, but I assert that they are true for all sorts of memorization, especially stuff that involves language, like memorizing speeches, theater monologues, or a business pitch to room full of venture capital partners.
Not only have I spent a lot of time memorizing music, but the context in which I do it with a large chorus gives me an opportunity to compare notes and learning styles with hundreds of other singers memorizing the same pieces under the same circumstances. I've witnessed two things about this process
Everyone memorizes differently. When learning a piece of music, some people listen to it a hundred times in a row until they've gotten it into their head in the same way a pop song on the radio gets stuck in your head without you even really trying. Others read the score and analyze the patterns, intervals, and rhythms until they've memorized it more the way you might have memorized the quadratic equation in high-school, a combination of wrote and structured derived understanding. (I use the latter tactic almost exclusively.) And there are oodles of people on the continuum in between these approaches. I believe which approach you take is related to whether you are a right- or left-brained musician.
Everyone struggles with the same stuff. Despite dramatically different learning styles, it never ceases to amaze me that the parts of a musical piece (or a speech, or a dialogue) that are hard for me to keep in memory are the same parts that everyone else struggles with! (This can actually be quite frustrating in the case of a chorus which is in the business of collective memorization. You don't want all 100 people having a brain-fart at the same time, so careful attention must be paid to the places you know are tricky for everyone.) At the last minute, everyone is still cramming the same challenging sections, not because they are hard to perform, but because they are hard to remember.
This experience makes me think they must be some sort of quantitative measure of the Memorization Difficulty Factor (MDF) for a work. The MDF would be a number that describes how hard a given work (or part of a larger work) is to memorize. Perhaps it could be thought as “the number of hours a person of average memorization ability would take to memorize this and repeat it with 95% accuracy 24 hours later”. But obviously it would really be a relative measure, allowing us to compare the challenge of memorizing one body of work to that of another.
So one might find that the MDF of Hamlet's “To Be or Not To Be” monologue is 6.5, the “Nessun Dorma” tenor aria of Puccini is a 15.0, and the complete alto part to Bach's Mass in b-minor is a 57.1. And maybe that carefully crafted paragraph you wrote down to break up with your girlfriend gently is a 2.4, and that perfectly honed business elevator pitch is a 7.0.
Such a measure would allow people to map an MDF to an expectation of the challenge ahead of them when memorizing something. And for those of us committing large works to memory all the time, it'd be great extra information when starting the memorization process.