but the kinds of cards I've had in my deck have changed substantially over that time, so I don't put much weight behind this.

The easiness settings in theory would help adjust for changes in card difficulties, I think.

Can you give an example? What would be the front/back of the card?

There's hundreds in http://dl.dropboxusercontent.com/u/182368464/gwern.cards.7z but it's literally what I said.

Or what would cue you to think of the quotation in the first place, especially if you have several from the same source?

Why does anyone think of a quote? It comes to mind as a convenient way to express some sentiment or situation. An example from a few minutes ago on IRC:

14:56:31 < X> how are you doing today?
14:57:28 <@gwern> X: feeling a bit put out that I sort want to go for a walk but I sort don't
and I no longer have the dog which would have pushed me over the edge in favor of taking a walk
14:57:44 <@gwern> 'those barbarians were a solution, of sorts'

A paraphrased quote from Cavafy's "Waiting for the Barbarians" encapsulates my vague frustration at the moment over losing my dog (died Thursday) as a commitment device for getting some minimal exercise.

What is there about my dog or daily walks that relates to Cavafy? Nothing, but it's a favorite poem of mine and there are quotes in my Mnemosyne and so it comes to mind.