I think the ones that make me smile most are those where original poster claims to have no interest in topic itself, and where a lot of other posters line up to post that they are also "not interested".
"Water sports" is one of these that recurs from time to time.
OP: "Of course, I'm not interested myself, but wonder what other people see in it?"
First Reply: "I wouldn't pay a bird to piss on me"
Second Reply: "+1"
Third reply: "Don't get it, completely pervy"
Sometimes...buried in all the completely useless replies telling us various people are way too straight to ever consider trying whatever it is...some poor sod actually gives a useful informative reply for anybody that does fancy trying something different. (Say "poor sod", because useful reply often triggers off another burst of "Jeez whatever made you do that?".)