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.
Harsher in Hindsight: Young man who believes in victory and glory, the homefront who is believing that they can win the fight: In the 1930 film, it becomes very apparent that the war spirit has not died yet.
Ho Yay: In the 1930 movie, in the scene where Paul is comforting Kemmerich after Kemmerich loses his leg, is very heartwarming, and also very shippy.
Do not forget the relationship between Paul and Kat.
Lighter and Softer: Compared to Johnny Got His Gun, another famous anti-war novel by Dalton Trumbo; this has led to many schools using All Quiet on the Western Front in curriculum over Trumbo's version due to Remarque's novel (in spite of its graphic imagery and downer tone) being less soul crushing and disturbing than Trumbo's novel. Also, Remarque was a famous critic of a totalitarian regime while Trumbo was a supporter of one, and Trumbo's work overplayed its hand beyond what was supported in history because it made the (plenty troubled but still democratic) Western Allied armies look worse than the German military under an authoritarian police state did in All Quiet.
Magnum Opus: Widely thought to be Remarque's greatest novel.
Narm: In the 1979 film, the scene in which Paul catches a random soldier from the other side and stabs him...and proceeds to watch him for ten straight minutes (in real time) while the man just lays there dying with a goofy expression on his face. Possibly a commentary on how it was a fellow human being that is being killed for a pointless war, but still absurdly executed nonetheless.
The horrors of the war are depicted in a brutally realistic fashion. The fact that this stuff actually happened should be enough to terrify anyone. There are descriptions of gas attacks and soldiers suffering from injuries, such as their faces being blown off. And then there's the fact that your friends could die any day....
There's a brief scene where a hospitalized soldier attempts suicide by shoving a fork into his chest, around where his heart is. Since he hasn't got enough strength to drive it deep enough, he proceeds to take a boot and use it as an impromptu hammer. Fortunately, he is restrained before he can damage himself too badly.
A Real Life example for the 1930 movie. When it came out, people still thought World War I was the War to End All War. Then another one broke out ten years later. So the young boys who saw this when it came out, thinking that the world will never have to experience such horrors ever again, grew up to be the young men who fought in an even more destructive conflict. Once this fully sinks into you it is heartbreaking.
Seinfeld Is Unfunny: The themes can look dated, because many other war novels and films repeated them.