Michael Ruffin sums it up rather nicely:

First, Advent is a particularly Christian observance. You are likely thinking, “Now, wait a minute—Christmas is a Christian holy day.” Of course it is. But Christmas has also been co-opted by the culture at large so that for many people the secular and commercial aspects of it are much more important than the religious aspects….

Advent is different. There are no secular Advent carols; there are no Advent presents; there are no Advent sales; there is no countdown of shopping days until Advent; there are no Advent television shows or movies….

Second, Advent encourages an alternative to pre-Christmas frenzy. One of the reasons that the observance of Advent runs so counter to the flow of our culture is that it is all about waiting, an activity at which our society is not practiced and that it thus does not embrace. Our Christmas practices, fueled by unrealistic expectations and by barely bridled materialism, lead us to approach Christmas at a frenetic pace that results in exhaustion and disappointment.

The observance of Advent, on the other hand, reminds us that life is not finally about what we do but about what God has done, will do, and is doing…. Advent slows us down and teaches us to pay attention.