A number of Christians have been expressing offense (or even outrage) at the use of "Happy Holidays"; instead they want people to say "Merry Christmas." There was discontent at the First Couple's holiday card, which did not specifically mention Christmas. Some have stated that they will refuse to shop at any store that has no specific mention of Christmas.
For those who are used to being in the cultural mainstream and having most of the surrounding cultural discourse support their cultural practice, it can be disconcerting to become de-centered. To suddenly become merely one religious/cultural tradition among many, with no special status, seems to connote disrespect.
But only if you are used to being in the cultural mainstream. For those who are accustomed to lack of cultural recognition, the intensity of the outrage seems odd.
No one is claiming that there isn't a Christmas tradition for some people -- merely that it is part of a broader winter holiday season that includes New Year's, Hanukah, Kwanzaa, Solstice, Ramadan, etc... But that lack of privilege, the lack of exclusive recognition, the inability to ignore other traditions -- that is what is most likely the galling aspect.
It strikes at the heart of the key difference in viewpoints: Do we acknowledge and respect diverse practices and cultures, or do we ignore cultural diversity and recognize only the dominant majority?
For those in the dominant majority, there is always the option of ignoring the minority traditions. Those in the minority, however, have no choice but to confront the dominant traditions in everyday life (e.g., Christmas specials on TV, etc.) and to be reminded of their marginalized status. Those in the majority may not even notice these references to their cultural traditions -- they are just "normal" -- but they may become deeply upset at the hint of removing any of these references. We don't notice the culture when it coincides with our worldview, but we notice any perceived "loss" of these cultural supports.
I believe in acknowledging and supporting diversity. I believe it is worth sharing the stage, instead of hogging the spotlight. I recognize that people find meaning in a variety of cultural and religious practices, and I am willing to support their path as long as it doesn't harm others or exclude others' paths. I'm not offended when someone wishes me "Merry Christmas", or "Happy Kwanzaa", but I am sensitive to the experience of being unrecognized and marginalized. And so my reply will typically be "Happy Holidays," with that meaning that you should find joy in the holidays you celebrate. And if you take offense at that, I think you have missed the point.