My dad told me once that it's generally known that it's impolite to talk about politics or religion with people you don't know very well. When I asked why, he told me that people have strong opinions about those subjects, so it's better to not talk about them at all to avoid an argument.
The internet has changed a lot of social rules, and politics has become a pretty open topic of conversation. People root for politicians the way people root for sports teams and you'll see the full range of opinions, emotions, and passions attached to those declarations of support. I hardly ever hear anyone complaining, online or off, about someone being 'too political' or bringing up politics as if it's a taboo subject.
Well, it's gotten a bit better but it's still considered an inappropriate topic to talk about.
Let me clarify - it's inappropriate for ME to talk about. Because I'm an atheist.
Religious people feel comfortable plastering their worship/praise/general ass-kissery of god all over the place - on facebook status messages, in chain emails, in forums, wherever. The religious are becoming more and more free with their expression, but it seems when an atheist friend or associate responds to such a declaration, they're still seen as being invasive and rude.
So it's OK to post "God is so awesome! I feel bad for anyone who closes their heart to feeling how loved I feel!" - that is no longer taboo.
However, if you were to respond and say, for instance "That's crazy, because I think living without god is awesome and I wish you could feel how it feels to be free of religion!" - YOU ARE A TOTAL ASSHOLE.
...Ok, maybe that's a tad dramatic, but my point remains.
Making ANY subject taboo to talk about creates problems for people and communication between different groups. A chunk of religious people have a ton of misconceptions, sometimes bazaar and sometimes pretty insulting, about non religious folks and to a degree non religious folks tend to guess the possible 'believer category' that the person their dealing with falls into, at times incorrectly assuming things about the person that aren't exactly true when discussing religious topics. This leads to a hell of a lot of bullshit fights that would be reduced by quite a bit if we could all just drop the whole 'not supposed to talk about it' attitude.
So this is my new vow - if I run across someone who is saying anything about religion and I feel like commenting, I am going to. Not to be rude, not to be obnoxious, but because if they're going to talk about it, I'm going to talk about it too. I figure if someone feels compelled to email me a story about Noah's Ark, they're showing that they are rejecting the taboo of not talking about religion and are open for a conversation.
Best case scenario - we both learn something more about what the other person believes. Worst case - they quit bothering me and I no longer have to delete annoying mass emails filled with Family Circus and Love Is... comics and clip art from Windows 95. So really, it's a win/win.