Because I have become increasingly Christian in the past five years, I have come to find Christmas a more and more offensive holiday. In terms of actual holiness in the Christian calendar, there are a number of days that are far more significant than Christmas. I choose to celebrate Christianity on those days.
Whatever Christmas may have once been, it has become the high holy day of Capitalism; and, of all the aspects of the capitalist religion I dislike, I dislike its sacramental manifestations the most. I do not believe that Christ would, at this point, want His name associated with an orgy of greed and waste where we venerate the Coca Cola mascot for a month. Tearing open shiny paper to reveal consumer goods is not a sacrament I want to have a part in. I'll choose eating flat, salty, flavourless crunchy pancakes over that any day.
On a side note, I also go mad in December being forced to listen to the same twelve vile songs on every single restaurant and store's PA system for five weeks. I don't like the songs in the first place and by Dec. 27, I'm ready to scream.
Furthermore, if one can somehow argue that taking money we might otherwise donate and spending it on goods people don't need and may not want is still Christian in some twisted way, our emphasis on Christmas over Easter really privileges the account of Christ's divinity in the Gospel of John over that in the other gospels, especially Mark. I'm not too sure that's something I want to be part of either.
Finally, what bothers me most about the two main sacramental events in the capitalist holy calendar (the other being Hallowe'en) is their direct appeal to children, using parental guilt and the threat of disappointed children as a means to force participation in a holiday more and more people are questioning.
EDIT: I'm kind of on Kahuna Burger's side but I would suggest that if one buys Mark's theory of divinity over John's, there is no need to celebrate the special day in May that the church believed Christ was born on but decided not to celebrate Christmas on.
And while I agree with Diaglo that Christmas was mainly designed as a competitor to/synthesis with the pagan festivals, first, of the Saturnalia and, secondly, Yule, a third century Roman might be every bit as baffled and offended by the thing it has become.