Infandous: What topics will they refuse to speak about?
Politics. Which seems unusual, I know, considering the nature of the book and his involvement with a revolutionary group. Prior to his experience in Cafe Musain, however, Marius would have gladly spoken of his political views -- -- As a youth, he is clearly defined to be a proud royalist, and undoubtedly spoke largely of it whenever he was attending to guests or being attended to as a guest himself. After finding out about his father's political views, that is, him being a Bonapartist, he obviously easily switched and talked largely of the topic; We see this when he debates with Combeferre during his first ( and only ) visit at Musain.
However -- and this should be noted -- immediately after the debate Marius realized that he didn't know where his political views were. I forget the exact quote, but he essentially realized that the views of his fathers were too old, but also that the views of Les Amis were too forward. Marius did not fit. And while that does not seem to be a big deal, well, it must be remembered that this depressed Marius. For a boy having to be thrown out of his home because his new political views did not fit in, realizing he did not even fit in with men of his own age is kind of a terrible punch in the gut.
Shortly after, he resolved to not be political at all, relying instead on his own personal views to decide whether something is worth supporting or not. In a way, he could be said to be neutral in the most base definition of the word, but nothing more, as we definitely know that he ended up supporting the views of his friends at the barricade. Regardless, politics is an uncomfortable ( ish? ) topic for Marius, and he prefers not to talk of it at all.