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: My Little Pony: Friendship Is Magic S2 E3 "Lesson Zero"
Fridge Horror: Celestia appearing at the end, shouting Twilight's name in anger, takes on a whole new meaning after watching Equestria Girls: having had one student fall to the Dark Side, she likely thought that Twilight was in the process of doing so as well and was probably ready to fight Twilight, to the death if need be, to protect Equestria from another rampaging, powerful unicorn.
Fridge Brilliance: Maybe that's why her demeanor changed so dramatically between arriving and the scene in the library. If she thought Twilight had gone evil, she would have had to put the fear in her. When she discovered only a scared, defeated unicorn, she changed back to her gentle mentor attitude.
Growing the Beard: As great as season 1 was, some episodes suffered from the need to shoehorn Twilight into a major role, so that she could write a letter at the end. After this episode, the writers had far more freedom in which characters they could focus on.
In addition, Twilight's own character based Aesops were less defined than the others previously, being based mostly on her (obviously temporary) Fish out of Water status. This episode would develop her defining vice as The Finicky One, giving her a personality arc like the other ponies that would recur throughout later episodes.
The realization that, in a later episode, Applejack wrote a letter to Celestia about how she DIDN'T learn a lesson, making Twilight's paranoia significantly more ridiculous.
Remember the lesson about taking concerns seriously? The first lesson of Season 2 to be addressed in the usual Every Episode Ending fashion? Gets thrown out the window in the finale.
Jerkass Woobie: Twilight Sparkle's Super OCD tendencies are upped to destructive levels in this episode, with her excessively badgering others (and later even trying to brainwash them) over an extremely trivial task. However given how much her neurotics have convinced her she will lose everything with just one screw up (to the point of having a nervous breakdown over it), you can't help but feel sorry for her, especially when it looks like it might actually come true.
Nightmare Fuel: One of the larger entries for the series so far. Even had its own page for a while. While it's hilarious seeing Twilight descend into insanity, it is also very disturbing. She not only makes some very unsettling facial expressions, but you worry for the safety of her friends when she decides to resolve the problem using magic.
Shipping Goggles/Fanfic Fuel: Big Macintosh runs off with Twilight Sparkle's doll at the end. While shipping the main character with The One Guy was excuse enough, this moment for some reason has inspired nearly every Twi/Mac fanwork since the episode aired.
All it took for Twilight X Rainbow Dash to explode was one line:
What Do You Mean, It's for Kids?: Between Twilight's major breakdown and Fluttershy's bear beatdown (which includes a neck snap), one sort of wonders why this episode didn't jump up to at least TV-Y7, if not TV-Y7 FV.
It's a good demonstration of what they can get away with without the E/I restrictions.