it really depends on the person/situation but in general i try to avoid religion and politics. my reasons for this is that most people seem to get defensive and argumentative when you talk about these subjects so it's difficult to have an intelligent discussion about them. sex is iffy. normally i try to stay away from the topic with people i don't know, but there have been a few times where the subject has come up and i've had very amusing conversations about them with people i had never met before.
also, personal things like ADHD, AS, that kind of thing are generally best left for your close friends and family. I'm more open about my ADHD since there's less of a stigma associated with it but i only bring it up if it's relevant to the conversation and if i feel the people around are the kind of people i feel comfortable telling about it.
and of course you should stay away from anything shocking or gross, unless the people around you bring it up, and even then be careful to sensor yourself because even when people bring up disturbing subjects, they don't necessarily want as much detail as you may be able to offer. better safe than sorry.