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.
Crowning Moment of Heartwarming: The very ending. WELCOME HOMЭ! DieBuster shows this scene from the Earth's point of view, with all the cities going dark and then lighting as the Noriko and Kazumi appear as two pinpoints in the sky. And then you blubber like a baby all over again.
Also, the opening to the first three episodes and the ending to the fourth, Active Heart.
HSQ: It is commonly held by fans of the series that once the Gunbuster activates in Episode 5, this gets knocked up several notches.
Nightmare Fuel: The series' exploration of Time Dilation becomes steadily more nightmarish as the story progresses. Noriko's best/only friend is the same age as her in first ep, by the last ep that same friend has a daughter graduating high school, and Noriko has only aged a couple months at most. Directly noted by Noriko when she realizes that this is why father always returned for her birthday; he didn't want to literally miss his daughter growing up.
Tear Jerker: Oh sweet God in heaven, the last episode will drain gigantic amounts of Manly Tears from you if you have anything resembling a soul.
The Woobie: Noriko early on, after she gets selected for the space mission. Poor Noriko - little to no self-esteem to begin with - suffers so much social abuse that she has a short breakdown not long after and ends up begging the Coach to rescind her selection... And this makes the sheer Badass later on so much more satisfying.