A third subject, apart from religion and politics, I would propose never to discuss is: hip hop. Do not discuss hip hop my friends! I'm serious.
Why? Because in hip hop there are TOO MANY different factions such as:
the old school vs the new school,
the East Coast vs the West Coast,
the gangsta rappers vs the backpackers,
the underground cats vs the mainstream cats,
The Stans vs the haters,
the people who think hip hop is dead vs the people who think hip hop is still alive,
I like Jay-Z vs Jay-Z is part of the illuminati etc. and all other types of crazy shit.
I have witnessed and have been apart of many heated discussions about hip hop. Both in person and online.
Topics like "Who are the top 5 rappers of all time, dead or alive?", "Who are the nicest rappers in the rap game now?" and "Who is better: Biggie or Tupac?" are all interesting topics but they don't end very well. As a matter of fact, they all turn out pretty ugly just like discussions about religion and politics.