I could go on about the characters, the sad little tree, the commercialism of Christmas or the wonderful Vince Guaraldi soundtrack. But I won't.
I could also comment on the religious aspect of the show - since it's one of the rare instances of Christian propaganda that seems to be accepted and tolerated by even the most ardent believers in separation of church and entertainment. But I won't go there either, even though I just did. ;)
I could trace it's broadcast history and talk about how it's been edited (or some say "censored") over the years, but you can get all that on Wikipedia. I just wanna share some great old TV Guide ads and thank the late great Charles Schulz, Bill Melendez and the aforementioned Mr. Guaraldi for creating a simple, wonderful half-hour of animation that has defined how people think about Christmas for the past 45 years. In the words of Lucy Van Pelt, "Charlie Brown is a blockhead, but he did get a nice tree".