Thank you @Toafactory for posting your opinions and thus allowing me to post mine without technically double-posting.
Since Toafactory got spoilers out of the way, I won't be hiding any in my analysis/critique/review/thing. Although I will be dropping a TON of proper nouns for those of you not familiar with the series (don't know why you'd be reading this if you weren't but here goes!)
First off, let me say that I have never wanted to personally murder a fictional character as much as I have wanted to murder Starlight Glimmer. Her actions in Cloudsdale are downright vicious, and borderline sadistic. She as also become a Mary Sue character, it's like the writers needed someone who could go toe-to-toe with Twilight, but Discord and all the other normal villains were unavailable. Now, I can understand Sombra, Discord, Chrysalis, Tirek, Nightmare Moon, and maybe even the Dazzlings or Sunset Shimmer being as powerful if not more powerful that Twilight. At least with Sunset Shimmer we know that she studied under Celestia. But Starlight Glimmer? She came off more as some backwoods unicorn than somepony who would be a real match for Twilight in a fair fight. Yes, in the season opener she has a spell that lets her steal the Cutie Marks from other ponies, but I have a hard time believing that she could somehow rewrite a spell by StarSwirl the Bearded EFFING STARSWIRL THE BEARDED!!! The only other time someone successfully altered one of his spells they became an Alicorn Princess!!! And learn to fly with unicorn magic and be a match for an Alicorn Princess. Starlight Glimmer is too Over-Powered, and even in her "reformed" state she poses too much of a threat to Equestria.
I felt that her motivations were poor, and the "redemption" was too quick and poorly handled. I would have much rather preferred Twilight to have used the spell to go back in time to confront Starlight after the season opener, and then take her forward in time to see the disastrous results for all of Equestria, rather than a quick bit of seeing Starlight's dime-a-dozen villain past and then having that quick "come to the good side" speech.
I do like the fact that Twilight had to rely on someone other than herself to save the day, but there is a big problem here. Since her appearance, there has been nothing redeemable about Starlight Glimmer. There is nothing in her or about her that could reasonably lend herself to the Mane Six. Discord at least has Celestia to vouch for him, and Luna was being possessed by an external force (I'll get to Sunset Shimmer later). There is nothing likable about Starlight Glimmer.
let me re-phrase that, There is nothing likable about a heroic good-guy Starlight Glimmer. Literally all of Starlight's "likable" qualities are things that much better suit a villain than a hero. Her deviousness, thirst for power, and her ability to trump the heroes. Here's a comparison: I "like" things about Heath Ledger's Joker in the Dark Knight. He's smart, unpredictable, devious, and acts as a perfect foil to the hero: Batman. Inasmuch the same way that the Joker only works as a villain and should not be redeemed, so I view Starlight Glimmer. I wouldn't like the Joker as a hero or do-gooder, because there is nothing in him that lends himself to the cause of good. I see nothing in Starlight Glimmer that can let her be the hero, and to finally get to that comparison with Sunset Shimmer, Starlight's wrongs are much worse.
Sunset Shimmer wanted to have the power of an Alicorn Princess, but Celestia banished her to the Human World where she plotted revenge. When the time was right, Sunset stole Twilight's magic crown to enslave an army and get revenge on Celestia. Sunset gets defeated, and offers to repent.
Starlight Glimmer lost her only friend when he got his cutie mark before her. Being too scared to make more friends, she eventually somehow learned to steal ponies' cutie marks and made a town of equality where she rules as dictator with an iron hoof and silences anypony who disagrees. Twilight and friends show up, liberate the oppressed masses, and fail to catch Starlight Glimmer who then escapes, and plots her revenge while stalking Twilight and studying magic. Starlight makes her triumphant return, undoes five years (our time) of friendship, all the while laughing at Twilight's suffering . Eventually, they come to an impasse and Starlight offers her repentance.
In both cases, Twilight points to herself, Spike, and the rest of the Mane Six as the gate to friendship. Here's the difference. Sunset's crimes weren't against Twilight, not personally. She was more focused on raising an army and going to war with Celestia. Something we can all understand. Sunset simply wanted to beat Celestia and prove herself better than the princess. Starlight, didn't want to just beat Twilight, she wanted to make Twilight suffer, and Starlight was enjoying the suffering that she caused. Rather than just spread her ideals of equality and fight Twilight in trying to prove one side right or wrong, Starlight elects to go back in time and cause untold suffering not just to Twilight, but to practically millions of ponies, for the sole sake of not "eye for an eye" or "cutie mark for a cutie mark," but rather a more evil sensibility of "Eye for a face, hand for an arm, You took away my town so now I will make literally everyone else in the entire universe suffer just to get back at you." Starlight's sins are personal, not just against Twilight but for all of the Mane Six and Equestria. Her heel-turn redemption happened much too quickly, and even if the rest of the Mane Six or Equestria will never know what Starlight did, but Twilight knows and more importantly: I KNOW. You can't just throw the equivalent of retconning FIVE YEARS out of continuity at me and expect me, the viewer, to be quick to forgive.
I'd like you all to imagine this: if in 2006, the Piraka weren't after the Mask of Life on Voya Nui, but rather they got the Mask of Time and went back to before the Great Cataclysm and ensured that Vakama and the others didn't become the Toa Metru, and that the Toa Mata never arrived. Or, if Kylo Ren in Episode 7 went back in time to stop Luke from getting C3-PO and R2-D2. Further Imagine that you come home to find someone who exposits to you that they are going to turn back time and make it so that you never meet your friends, they say that they are going to make you and everyone you love suffer, simply because they enjoy your suffering.
I don't buy Starlight's "redemption" for a second. With Sunset Shimmer, we at least had her out of sight and out of mind in the human world. But, what's going to happen to Starlight? I don't trust her going back to her village, living with Twilight, or roaming Equestria.
Now, we have had two more films to see Sunset Shimmer work out her redemption and earn her place among Twilight's friends. We may yet in the future have instances for Starlight Glimmer to truly earn her place. I don't trust Starlight Glimmer, not yet.
I do like the concept of "Everyone can be redeemed, even if they don't deserve it." But the execution is poor. I would have rather had Starlight pull a Winter Soldier, leaving Twilight and contemplating both sides of the story and eventually coming to terms with the truth and returning to Twilight later in Season 6 with a true repentance, rather than this 180 on the spot turn.
There are always those fans who cry "Jumped the Shark" after every season, and this time I am in that camp. Time will tell. I may be wrong, which in time would lead me to change my opinions. But for now, I say that Starlight's redemption is my only issue with the season closer, and I currently have her in the "Characters who I wish didn't even exist" category, right next to Flash Sentry. wait a minute... heh. I ship it. Sorry for the long rant, I guess I just needed to blow off some steam. I may be taking this too seriously, but this means a lot to me.