From a musical background, I will say this:
I cannot stand live performances where the choir or backup drowns out the singer of the carols. For example, several performances on Carols by Candelight (that's in Sydney) the other night had the singer being drowned out by the choir. Particularly during "Mary Boy Child" and some other song I can't remember.
On a sub-note, cases where the choir and the singer are out of time. It sounds absolutely horrible.
One thing did make me giggle though: during the show, the guy who arranged and conducts the songs (NOT the choir) is somewhat well-known as he writes/arranges music for shows like Australian Idol and Dancing With the Stars (and variants of). He also ends up NOT conducting the Hallelujah chorus. Instead, he takes the cymbals (the hand ones) and ends up doing one of the following:
-Wandering through the aisles singing along and wielding his cymbals waiting for his big moment.
-Standing on stage, with the cheesiest grin on his face, waiting for this big moment.
This year, they went with the first one, which had me in hysterics over how much of a big deal it was for HIM and for the audience (given there's only ONE cymbal crash in the whole song).
Other things I noted on Carols By Candelight:
-Little Drummer Boy (which I hate) sounds horrible as a duet.
-The "Legends of Aussie Rock" (half of which I'd never heard of) did a fairly "adult" song, which made me laugh (and also sailed over the heads of the audience)
-One girl resembled a walking Christmas tree....picture flaming red hair and emerald green dress.
And for those who have no idea what I'm referring to, Carols by Candelight is a charity event done every year in various states with the money going towards some charity (the one I've mentioned goes to Royal Society for the Blind). It involves a list of Aussie (mostly) celebs who are dragged out and made to perform carols for the audience.