So there's this chord in this arrangement of "O Come All Ye Faithful" by David Willcocks that is apparently revered throughout the choral world. As a grinchy half-Jew I didn't get the appeal and wanted to understand it, especially after I noticed that it's a half-diminished chord, which also shows up in Mariah Carey's "All I Want For Christmas is You." That chord was the subject of a dubious Vox explainer video a few years back that dubbed it the "Christmas Chord" (lol). I wondered to myself: is Christmas music really so harmonically bereft and emotionally narrow that a bog-standard half-diminished chord becomes notable in that context?
(I might be kind of bitter about Christianity's stranglehold on choral music.)
So, at least in the context of the Willcocks arrangement, I might be selling the chord short by calling it bog-standard. The chord itself is whatever, but the way it's approached is cool, by setting us up to expect a G major chord and then giving us something totally different.
It's not even immediately clear what the chord is supposed to be until the next bar, when it becomes apparent that we're in A minor. It then winds its way back to G major by going through E minor. It's a very clever reharmonization, hard to pull off.
It's easier to see what's happening if you compare it to what happens in the first verse, which is super diatonic and boring. Compare that to what happens in the third verse, where it's easy to be distracted by the florid descant (top line), but here we have the E minor reharmonization too, which sets us up nicely for the big WORD chord reharmonization in the last verse. In other words, it's the context of the chord that makes it cool, not the chord itself, though you wouldn't know that from the memes and the t-shirts.
If you're like me you see that chord and you're like "what's the big deal?" The deal is that the restricted tonal language of the Christian hymn is gradually opened up until you get this brief glimpse of a more lush musical world. I do think there's something particularly Christian about this attitude toward music and toward pleasure in general, which is all about denial of earthly delights to get your eternal reward in heaven. For example, this Catholic journal calls the chord an "illicit thrill," which seems telling.
It's not something that particularly resonates with me musically or personally, but I have to acknowledge that it's really artfully executed in that context!
But yeah, half-diminished chords are so common in some other genres that they're just not notable at all there. Blue Bossa, a tune that every jazz musician is sick to death of, has half-diminished chords and it's no big deal.