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 Music of Awesome: Both the 1st OP: "Sympathizer" by Minami Kuribayashi & the 2nd OP: "tRANCE" by GRANRODEO, as well as the hard rock music that activates during battle scenes.
In episode 12: Excel's reaction to Steiner's death. It's something to watch her emotional outburst, then see her crumble into tears when Steiner's copy flickers out of existence after carrying her to safety.
Excel: (tearing up) "Steiner? Steiner?! No! Where did you go?? DON'T LEAVE ME!!"
They Changed It, Now It Sucks: The inverse. The anime changes the story from the manga significantly. Most people liked the changes since it made Keita more likeable and Kuro more successful in battle; though a few others not so much.