Christianity has a very simple central premise: we believe that God loves humankind so much that he became man as Jesus, died for our sins, and came back to life, thereby conquering sin and death. He thus reached out to us, because we are incapable of reaching out to him. Anyone who trusts in Jesus will have eternal life. Christians believe that most of the Jewish Bible (what we call the Old Testament) foretells this; the main theological fight between Christians and religious Jews is over the identity of the Messiah (Hebrew Moshiach; Greek "the Christ"). We think he already came, with a mission different from the one they expected.

I think Christianity is controversial ``around here'' (cyberspace) partly because it's the majority religion in the West, partly because of the fights between scientists and Christians (based on misunderstandings all around), and partly because conservative politicians have tried to politicize the religion:

Being the majority religion in the West, it gets blamed for the general human tendency to repress minorities. People repress people; look at Cambodia, Bosnia, or Rwanda.

Science vs. Religion: many naive Christians make the mistake of thinking the Bible is a science textbook (note to my creationist friends). Many scientists make the mistake of trying to apply scientific philosophies to ethical and spiritual questions (i.e., non-physical questions), where it just isn't applicable. I don't have to do this, because I'm a dualist!
The Lutheran church once ran an ad that said ``Jesus came to take away your sins, not your mind.'' That's what I think too.
(Recently I've heard biologists making obsolete arguments based on the clockwork universe concept.)
Even more recently, I've started trying to write a general explanation of my views on this whole topic.

Politics: Jesus said give all your money to the poor, which is unlikely to become a plank in the Republican Party's platform; he never said anything about abortion. He also said to pay your taxes! And that was to a pagan foreign oppressor, to boot.
(Note that I'm not trying to claim that Jesus would always vote Democratic; I'm just trying to counter the frequent implication that he would always vote Republican.)

As one example of these kinds of misunderstandings, you sometimes hear a quote attributed to Tertullian ``I believe it because it is impossible''. This turns out to be a misquote, and taken out of context at that! See point four in this list of Tertullian quotes.