Man, I really don't know how to tackle this one, so much I want to talk about and so little time... I guess I'll make it on a list format like Stricker used to make and call it a day.
What I liked:
* Alternate timelines: Did it serve to the story? Sans maybe the last one and one of the others (any of them would do, but I'd personally take the Nightmare Moon one because it's the nearest disaster to the time Starlight traveled to) it really didn't. Was it blatant fanservice for the most part? Yes it was. Did I like it? hell yeah I did, I was geeking out during a lot of them. And while sadly both Discord and Tirek got the short end of the stick there is so much you can do with 44 minutes. Admittedly, the enjoyment you might get out of this depends heavily of your suspension of disbelief since time travel stories tend to be a mess and this one is no exception (everything happens at the same time and what affects all this changes is how Dash fails to pull off the rainboom, despite the rainboom never happening some timelines have the mane six with the normal cutie marks, it turns out Flim and Flam were the Dr.Eggmans of this setting after all (sorry, I'm being obscure and referencing and old as hell review I did), etc)
* Humor: Haber is a very hit and miss writer in this regard considering he wrote things like Simple Ways but when he nails it, he nails it hard! This chapter had so many funny moments that I can't really say I wouldn't rewatch it over and over again for the laughs it gave me at some points (and the alternate timelines fanfic fuel), parts like Twi awkwardly trying to make the events of the first Sonic Rainboom happen, or how she couldn't memorize her own notes for the class (even after several rehearsals), or how on the scale of bad to worse futures the Flim Flam brothers somehow rank between Tirek's rampage and the end of all life (yes, I'm pretty sure the Flim Flam thing was meant to be a joke guys) made me laugh quite a bit, the animators also had a lot of fun with it.
* Starlight's Defeat: I'm going to touch this scene multiple times since those last 10 minutes are a mix of all sorts of emotions for me so lets start with what I liked, something I mentioned I found very interesting on the season opener was that Starlight wasn't a villain the mane 6 had to overpower to defeat but rather outsmart her, and in a way it's the same on this episode. Twi makes very clear that she and Spike alone couldn't defeat Starlight nor made her change her ways just like that, our equalist unicorn had to make the choice by herself. It's a bit refreshing after the brainwash rainbow waves against Nightmare Moon and Sunset Shimmer and the friendship nukes against Tirek, Discord and the Dazzlings.
* No "Buy Our Toys!": I'm pretty sure Starlight is going to be a pony that will have some merchandise besides the one that it already exists but it feels so nice to have an ending that doesn't feel to be as merchandise driven as the others we had for a while. No royal weddings, no random princesses showing up, no eye popping transmutations to the mane six's designs (thank Celestia for that, there is a Cutie Mark Magic line and I was dreading when it would show up). While in the first case it was awesome to see how the staff at DHX managed to turn what many predicted to be a sacharine borefest and turned it into one of the most epic commercials I've ever seen the rest of the attempts were... Not quite as successful (disregarding the quality of the episode itself).
I have no strong feelings one way or the other:
* The Amount of Timelines: I mentioned before that while I enjoyed the alternate futures, for the episode to work we needed only one of them (and maybe two if you want to include the last one) I personally expected an alternate timeline that started with Nightmare Moon and progressively got worse as more villains showed up but alas, we didn't get that.
* Nightmare Moon's Kingdom: The very first villain of the show as well as the very first near omnicidal being who got away with an apology and a pat on the back (I'll admit I'm a lot more lenient with Luna since it was the pilot episode and those tend to be kinda weak)... Or so we thought... Nightmare's rule doesn't seem to be so bad (breaking Twi's speech about every timeline being worse), she might be harsh or even a dictator for all we know but the ponies live, the vegetation hasn't perished after a year of her rule and the living conditions of her subjects seem to be relatively nice, compare that with the lands filled with war and conflicts and the near inhabitable wastelands other timelines bring. But while I find this to be a very interesting thing to think about it feels like at the same time it cheapens a lot of things on the pilot,
* Starlight's Backstory: It was very petty, rushed, and doesn't justify even half the things she did. Yet at the same time it works because it follows the theme of the episode (the Butterfly Effect), Starlight even questions this because she can't think Twi's friendship being all that important that the future of the land depends on it (I guess there were other backup plans for the whole Elements of Harmony thing? Otherwise I'm sure everything would have stopped at NMM) yet as the Princess of Friendship states "It's not just my friendships that are important to Equestria! Everypony's are! When yours ended, it led us here". On the other hoof, rushed as it was, it was still better than every other redemption we had beforehand (Namely: Luna, Discord and Sunset's) it's not a compliment per se (and I'll get later on other differences between some of these) but you could say it's an improvement.
* Twilight and Spike's Excellent Adventure: After the premier I thought we'd be see more of the kind of two parters where the rest of the mane 6 contribute to the cause, yet this one was exclusively for the purple alicorn and the similarly purple dragon, the only reason I'm leaving this one here and not as a negative it's because the episode makes an effort to state that this isn't the ideal scenario, that Twilight alone can't fix this issue and that it was Starlight who ended up solving her own mess.
* Daring Do and the Forgotten Plotline: Okay, no Daring Do on this one, sorry. Something Season 4 did (up to you how well it did it) was to have one arc that spawned the entirety of the season and it affected both the opener and the ending, this seems to have been forgotten on Season 5 since it opens with Staright, who disappears and is nowhere to be seen (sans two hidden cameos) until the very end, at the same time, the Cutie Map quest line is about the mane 6 traveling in groups and solving conflicts in some communities yet none of this is relevant to the ending, I guess you could relate the finale's message about how deeply friendships can affect a community and look forward to the grand transformations that the Cutie Map quest locations will go through in the future, but it doesn't feel like there was much thought put into this (if there was any thought at all).I'm also chalking this one on the Neutral Zone because I can't tell if this is really a bad thing.
The Nerd Rage Corner:
* The Animation: I'm not one to usually complain about this (most of the time I praise it) but the combat scenes on this episode were jerky as hell, you just need to watch the way the royal guard and Celestia fight and you can tell the movements are not as fluid as they usually are (Dash and the Kujo Pie Sisters being the exception), the same goes for the mini fight between Zecora and Chrysalis.
* The Special Snowflake: Another thing I really didn't like about this episode is the way it paints Starlight as someone so awesome and gifted at magic that not even a princess (renown for her prowess at magic) could best her. Back on the premier we saw her struggling to keep up with the alicorn and raging about how things she took months of practice came naturally to Twilight, it really helped to differentiate the characters and gave a lot more impetus to Starlight's ideals about how someone being special or different was wrong and I'm sad to see all of that gone here, it would have also helped in her backstory since Sunburst discovering his talent for magic and later enlistment at Canterlot was what separated him from Glimmer.
* Starlight's Forgiveness: Okay, this is where the last 10 minutes of the episode fall flat to me. I'm going to compare this scene with both Discord and Sunset's redemption (as I said, going easy on the pilot even if it was as bad as this one). First is the draconeequs, both him and Glimmer's redemptions feel like a let down to me because both episodes (this one and Keep Calm and Flutter On) make a great job at showing why these two characters are such cool villains and both end up crushing my hopes of ever seeing them like that again. The other comparison (and the most important because there are more parallels between the characters) is with Shimmer, and yes, both redemptions were rushed as hell but there are 2 key differences here that separate the characters:
1) Sunset Shimmer was an awful villain, her plan was terrible and was filled with holes, her story felt more like a petty pony driven by jealousy, hatred and little else. Starlight on the other hoof managed to outsmart everypony she met, she was cunning and manipulative and her ideals of harmony clashed heavily with Twilight's making her a way better antithesis to the purple alicorn than Bacon ever was.
2) The Aftermath: Sunset had to deal with a whole movie (and some comics if you count them) worth of emotional beatings and trials in order to even be considered to be part of the main cast (even more the rest of the school), also, she was punished by her actions (albeit the punishment was a lot lighter than what she deserved). Starlight Glimmer got a hug, a musical montage and now she, the mane six and the ponies of equal town are all Best Friends Forever...
Hell, even Discord got a better arc if you take Season 4 into account (shame What About Discord? kinda threw what that season finale did out of the window).
* Mane 7: Okay, not really confirmed, for all we know she might be a secondary character like the local chaos god... But the addition of Starlight to the cast on the heroes makes me... Very wary about the next season... It's just that she seems to be playing a lot like a Sunset Shimmer replacement (since they want to keep EqG and FiM separated) and as I posted above she feels like a very bad knockoff of Bacon. Also since she no longer has this ideological clash with Twilight it feels like all of the appeal of the character is gone. It's just one of the things that keep piling up that doesn't really make me feel so hyped about the future (except the movie and some more EqG, I'm really looking forward to those), or if you want me to sound less like a drama queen, I'm just not looking forward to her hypothetical episodes as I'd do with Discord or Iron Will or Diamond Tiara (another character with a rushed redemption, but feels more interesting than Glimmer) between other characters.
tl;dr: I enjoyed this episode quite a bit but... That ending...