Oh God. I guess what gets me fired up the most is when atheists stereotype Christians as bigoted, ignorant, etc. (which is not to say that isn't true of some). I always want to jump in and prove that some of us can use both faith and logic, but you have to dodge a lot of barbs before you can meet in the middle.
Truth be told, I'd rather be arguing about things like which Mel Brooks movie is the funniest (it's Young Frankenstein), but those get lost in the noise, and when I see yet another "Why Do Christians Hate?" thread it's just oh, so tempting, so I usually just leave the forum.