I'm a pianist, but that's not why I say this. The piano is key (pun intended) in Christmas ballads. It's an absolute must.
A PIANO MOTIF
Now, this can be a few things. This could be the actual melody playing along with the vocalist. As Faith Hill sings "Where are you Christmas?" listen to the piano play the melody in the verse with her as she sings it. Because you know, Faith Hill probably can't sing a melody on her own.
The motif could be the melody of some part of the song being played during an instrumental break. Listen to Amy Grant's rendition of "Grown-up Christmas List." At 3:20 the keyboard samples a little bit of the verse melody to close out the song.
I'm scared of the girl at 2:11. Gracious.
And let's just not talk about how weird it is to watch Amy Grant try to sing "sexy."
This motif could also be a completely different motif that has nothing to do with the vocal line at all. But probably not. If you're writing a cheap Christmas ballad, you should recycle as much material as possible.
INTERVAL OF A SIXTH
Whether it's a major 6th or minor 6th does not matter. It just needs to be a 6th. These 6ths are played on the piano and in the upper register. 6ths are like middle school girls. They are full of drama. Throw them in your ballad and you've got everybody grabbing their Kleenex.
Listen to the 6ths in "The Gift" at 0:7 seconds.
Licks, runs, embellishments, ornamentation, whatever you want to call it. Mariah Carey and Whitney Houston are surely the queens of this. But others can try their hand at it as well. (And if you've seen anybody legit do licks, you know it involves their actual hand.) For Christmas ballads, they sure do add the cherry on top. And who doesn't want a cherry on top of their Christmas ballad sundae? Listen to Mariah as she sings "Hark the Herald."
Christina is also a pro in the lick department. And the grunting department at 0:17 seconds.
But sometimes licks don't turn out as planned. Take Faith Hill, for example. Go back up to "Where are you Christmas?" at 1:20. Listen for "rearranging" at about 1:25. I have no idea what is going on there, but I guess everyone in the studio was okay with it.
Regardless of your ability to successfully execute ridiculous amounts of vocal licks, you should probably include them in your Christmas ballad.
AND FOR GOODNESS SAKE, LET THE CHILDREN SING
"Christmas Time is Here"