Lyrics like those in "No Time To Cry" by Iris Dement.
Just about my favorite musical technique is the bassist playing inversions (playing not the root note of the chord being played by the piano, but one of the other notes in the scale of the key the song is in, typically the 3rd). For a sterling example of that listen to the bass in "What Becomes Of The Brokenhearted" (the original, sung by Jimmy Ruffin). Playing bass on that fantastic song is my favorite: James Jamerson.
Another is modulation (key changes), one reason I like Baroque-era Classical music as much as I do. Bach’s chord sequences/progressions are insanely great!
And of course vocal harmonies. 2-part is nice, 3-part even better. And 4-part? VERY hard to pull off, but Marty Stuart and the three guys in his great band The Fabulous Superlatives do it.