I don't care if there was three days to spend time with Discord. No way every single pony would enjoy the inside jokes that much and laugh that hard - the personalities are too different. I think it makes sense for Pinkie Pie and maybe RD (on the one hand she probably would trust Discord the least, yet on the other hand she has a strange sense of humor), but I don't see the rest doing that. We just had a Halloween episode were we just saw this where Fluttershy realized Nightmare Night wasn't for her in part because she hated how she scared her friends (even though they enjoyed it). Didn't she think it was wrong? I'm not sure about Applejack, but I really don't think Rarity would have been the type to constantly laugh at "inside jocks" like that either...
I won't argue about the quality of the episode, but I will address these points.
I didn't find it too strange that all the girls would have liked the inside jokes. First of all, even if you normally wouldn't find something funny, being around other friends who do will often make a shared experience funnier. That's often why inside jokes aren't funny to outsiders in the first place. Secondly, we didn't actually see what led to most of the jokes, so that aspect is left up to our imaginations.
As for why Fluttershy didn't find the use of inside jokes cruel, why would she? If the Warcraft movie references Leroy Jenkins, no one would think they were being mean to the audience unfamiliar with it. As far as Fluttershy was concerned, Twilight had a great time organizing books, and then showed perhaps an unusual amount of interest in the jokes they made with Discord. It didn't register to her how often Discord has been bringing up the jokes specifically in front of Twilight. There are also shades of "Lesson Zero", in that the girls once again underestimated how well Twilight adjusted to being sociable and how prideful she is.
Note that as soon as Fluttershy realized that Discord had been using the jokes to hurt Twilight, she and everyone else stopped finding them funny. Her briefly using the inside joke to get back at Discord falls in line with how she handles Discord in other episodes.
As for the potion inside joke itself, it was a case of taking what would have been a painful memory and turning it into a good one, though I definitely got the impression that they were playing it up to show Discord what he was actually doing to Twilight.
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Also, I'll address complaints I've seen on another board, one of which was that the message of this episode says that it's bad to be an introvert. Everyone was already aware that Twilight sometimes likes to spend a lot of time by herself and didn't begrudge her for it. The fact of the matter is, though, that no matter what you do, you'll miss out on something else--the lesson of "Too Many Pinkie Pies". The episode was more about the fact that Twilight refused to admit what was really bothering her, and did so out of the belief that she couldn't possibly be jealous.
I've also seen complaints about Twilight's character regressing in this episode to a season 1 state, which I don't believe. Twilight has always been an introvert, which is to say, that she generally finds it more relaxing to be by herself than with others. Just because we hadn't seen an extreme demonstration of this in a while doesn't mean it disappeared. In contrast to Moondancer staying in the library all day in "Amending Fences", Twilight clearly told everyone that she was organizing her books and how long it would take, and everyone was accommodating of this, besides the inside joke thing.
As for her obsession with understanding the inside jokes, again, just because we haven't seen her act like that in a while doesn't mean this aspect of her disappeared, and this time, it was clearly aggravated by Discord rubbing it in. Twilight has definitely become better at managing this, but it popped up a bit even in "Party Pooped". There's even a bit of continuity with "Lesson Zero" with how the girls went along with Twilight's suggestion to recreate the events leading up to the jokes, since it was important to her.
) from the future LOL. 
