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.
YMMV: The Dresden Fillies
Complete Monster: Trixie becomes this as soon as she lets herself get possessed by Nightmare Moon, and arguably even before that.
"Funny Aneurysm" Moment: You know, come season 3, Twilight's the one who, in-canon, if she and the other ponies were human, has broken the Laws of Magic (specifically the prohibitions against mind control, as per Lesson Zero, time travel, as per It's About Time, transforming others, as per The Cutie Mark Chronicles, and arguably, killing people, as per Too Many Pinkie Pies). At least by Magic Duel, Trixie's now a repeated Second Law breaker.
the passage in Chapter 10 of False Masks where Harry talks about what it looks like when a father loses his child is kind of hard to read in light of Changes.
Episode 5 of season 3 Magic Duel features Great and Powerful Trixie using a dark source of power, that corrupts her to get revenge on Twilight Sparkle. Pretty much the concept of Strange Friends sans Dresden.
We finally learn why the Order Triune is so deadset on destroying anything even remotely resembling Obsidian in False Masks 14.Obsidian's corpse after his first death disintegrated anypony who touched it as well as anyone who came in contact with the black vapor surrounding it. He then resurrected himself 50 years later and tried to take over Equestria AGAIN.