YMMV / My Little Pony: Friendship Is Magic S2 E3 "Lesson Zero"
Growing the Beard: As great as season one 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 for characters they could focus on. Also, 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.
Twilight's Sanity Slippage began when she realizes that she has not sent a weekly friendship letter to Celestia and therefore (from Twilight's perspective) she has failed Celestia. The Crystal Empire two-parter in Season 3 shows that Twilight's worst fear is indeed failing Celestia.
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.
Idiot Plot: While better received then most examples, the episode ultimately only happens because Twilight panics and overreacts to a minor problem, and the other characters only make a token effort to calm her down while otherwise brushing off her concerns.
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.
Narm Charm: Twilight Sparkle's Sanity Slippage provides a lot of humor without clashing with how scary and dramatically it's played.
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.
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:
Wangst: Played for Drama since the story and its consequences are the result of Twilight's overreacting to the unfounded prospect of being sent back to school by Celestia again. Also Played for Laughs since much of the episodes comedy is a result of it.