These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ho Yay: Between Hanno and Kai. Author Thomas Mann (who was a closeted gay) stated that the two boys stood for different aspects of his own personality. It's a safe bet that nowadays the subtext would turn soon into text.
Christian. On the one hand, he suffers pretty much and is disrespected even by the servants. OTOH, his neuroses and uselessness can get pretty much on one's nerves.
Tom. He's not a good father by any means, but it's hard not to feel for him; his siblings' uselessness in business and Gerda's unfaithfulness make his life very stressful.
Also, the English teacher who can't keep his class full of boys in puberty under control. At one point, he punishes Hanno for laughing, which he doesn't dare with the other boys, who are WAY worse. Hanno angsts how he can't even feel pity for his teacher, for this.
The Protestants are at church service and sing one song (not existing in Real Life), which is this. In German, it starts with "Ich bin ein echtes Rabenaas / ein rechter Sündenkrüppel" (last word meaning "sinning cripple" or "crippled sinner").** Not once, but twice the worker Grobleben almost spoils a festivity, when he's supposed to congratulate Thomas Buddenbrook and somehow manages to get the topic on the fact that "some are rich, some are poor, the one will get a polished coffin and the other one only an old box, but we all have to become moldiness..."
Values Dissonance: Nowadays divorces are considered differently, but at this time, they were a cause for big shame. Especially if a woman had two in a row.
The Woobie: Poor Hanno, of course. It is telling that the one situation where Hanno and his father feel closest is when Thomas shows being hurt and confused because the captain (von Trotha) "has been with mother for two hours".