There are two kinds of board games I love to play. The first has complex and subtle rules: they are difficult to learn, at first (your first round might feel like riding a roller coaster blindfolded, trying to understand what’s happening) but when you’ve mastered the gameplay, there are all sorts of subtle ways to apply the rules to win the game. (Think any of the modern “nerdgames” like Settlers of Catan, Dominion, or my current favorite, Wingspan.)
The second kind has simple to learn rules, but they interact in complex ways to reward you with a lifetime of play. (Think chess or poker.)
My favorite music falls almost universally into one of these two categories. In the first type, I think of the a cappella arrangements of Gene Puerling. On first listen, you might be disorientated by the harmonic shifts, the non-tonal reharmonization, and the challenging voice leading. But it rewards hard work with joyous singing and listening.
The second kind might be pieces like the Mozart Ave Verum Corpus, or even his Requiem. There is nothing particularly challenging for a competent musician to play or sing, but it rewards repeated work with ever deeper understanding. Robert Shaw said exactly this about touring with the Mozart Requiem: after six or seven weeks of nightly performances, the musicians had attained a deeper understanding than even they knew was possible–with the entire performing ensemble weeping at the last performance. I similarly remember an old friend talking about a study edition of the Bach B Minor Mass, which he kept on a dedicated stand in his office – every day he would flip a page, and every day he would discover something new about a work he already knew well.
When I look for new music to program, I often look for things that will fit into one of these two categories.