Reading this thread, I was surprised to see how this song was interpreted. I have never thought of it as dark or patronising before. It just seems...well...a song of hope?: looking to the future, in a way.
According to this source: There is a distinction between a Christmas carol and a Christmas song. A Christmas Carol has a fairly rapid, regular beat, which would, therefore, exclude a meandering, crooning song such as “White Christmas”.
I would guess, according to its etymology, that a Christmas carol is one that sung in a chorus, going door-to-door. A Christmas song is just a more general category? So "Grandma got run over by a Reindeer" is a Christmas song, but not one you'd really sing in a group at someone's doorstep.
I actually like the song quite a lot. The lyrics and tune are beautiful. It's just that one line that always sort of rubbed me the wrong way. Anyway, the real question here, is what is the difference between a Christmas carol and a Christmas song?
I don't find it patronizing either--actually, I never considered that until now, uselessness. But it is true, as ptero points out, that the original words were quite dark. Either way, it's one of my favorite Christmas tunes. :-)
It's worth mentioning that the original song, as it appeared in "Meet Me In St. Louis", was a wistful, melancholy piece. The lyrics we sing today are rather cheerier than the lyrics that Judy Garland sang (as you can hear for yourself). I suspect that the reason why the phrase "merry little Christmas" sounds so dismissive is that it's a remnant of the original cynical lyrics.