I feel like Tchaikovsky's music for the Nutcracker defined the mechanics of what much of Christmas music is these days. From the harmonies to the orchestration, a hundred years of composers, musicians, and listeners have grown up on that music, and it has clearly infused much of other Christmas music that has been written since. Every time I watch Home Alone (I was a kid in the 90s and have to watch it every year), I feel like John Williams was paying homage to Tchaik. I think what I like most about The Nutcracker, and really just Tchaikovsky, is his immaculate ability to orchestrate. The colors he achieves are glorious.
I will definitely be listening to plenty of the Nutcracker Suite this holiday season...music I very much believe is the "Christmas" sound. What a great piece of music from a great composer!