Hey Plum, I'm the one who just took my Christian History final. I'm also a religion major. I also grew up in a pretty religious family, and I taught Sunday School throughout high school.
I didn't have the time, (nor the patience) to read the rest of this thread, but I assume by now you've got most of the info, but I'll do a little bit.
December 25 is not a date in the Bible or anything. It actually is the date of one of the main pagan feasts. (I actually think most churches now believe the date was sometime in Spring, although you shouldn't quote me on that.) As with most (major) religions, when trying to spread Christianity, many things were fused with pagan holidays. Hence, even though the holiday is religious, the date on which it's celebrated is not.
I love the religious aspect of Christmas. God sent His only son to Earth to save us (and by us, I mean all of humanity, I do not hold the belief that all who aren't Christian are damned for all eternity or anything. You worship the way you want, I'll do it the way I want, I'll see you in Heaven is my philosophy...) Though I love Christmas, it has definitely become too commercialized for me. I like to give gifts that are meaningful to show someone that I love them, but getting the most expensive, new high tech toy for everyone and their dog really gets to me.
I just want to briefly mention the idea of the Trinity, which is central to Christianity. God/Jesus/the Holy Spirit are one (at least to Catholic and mainstream protestant groups.) They are both one and three (the both/and dynamic is central to Christianity) and of one "substance." Jesus is both human and divine, and is the "Word" or "God" made flesh.
For Christians the most important holiday is (or should be) Easter, not Christmas.
After Christ's death, many different sects were formed apart from traditional Judaism. The split between Judaism and Christianity was a painful one for both sides and didn't officially happen for many years after His death. It's pretty complicated, and since I don't have my notes/book with me right now, I'd prefer not to say anything too specific for fear of being wrong. I do know that there was controversy in early Christianity over whether or not one needed to be Jewish to be Christian.
The Old Testament (Isaiah in particular) tells of the coming of the Messiah. Christians believe that that prophecy (or promise, for most) was fulfilled in Jesus. The idea of the "second coming" came from the fact that the end of days was to follow the coming of the Messiah. Obviously, it didn't come as quickly as expected, which is where we get the "second coming," meaning Jesus will return to Earth and that will be followed by the end of days.
Sorry for my really basic theology, but it's complicated to explain (as I'm sure it would be if someone were to try to explain Islam to me on a message board.) Also, please note that there are tons of strands of Christianity, so all of my comments don't apply to all of them, but it was pretty mainstream theology.
Why do we play with fire?
Why do we run our fingers through the flame?
Why do we leave our hands on the stove, although we know we're in for some pain?
-tick...tick...BOOM!