Depression and/or my alcoholic mother.
Two subjects that aren't pleasant when brought up.
Those aren't usually brought up though, cause I'm a happy drunk. 
Yeah, I'd have to say if someone can get me on the topic of the disregard my folks had for me growing up, that usually puts me in a down mood. My grandfather--the topic makes me both happy(he was an incredible man) and sad(he left far too soon from my life--when I was 19--and I still needed him given the first topic that puts me in a funk).
As far as worldly stuff, when I'm with other adult women, I can't stand when all they talk about are their children and mommy stuff. I get bored...fast. Maybe it's because I'm immersed in 'mommy stuff' all day.
As far as the big stuff. I stay away from just about any topic with people inclined to turn it into a personal attack if engaged on something. A lot of wasted energy and it's just going to generate bad feelings in the end.