@Beau Skunky  
Sunset Shimmer was hated initially because she was introduced in a badly written movie, EG1, and the few good parts that included more then one-dimensionality from her character were only half complete with half a story or less.
 
EG2 had really good writing and really good progression of Sunset’s character and personality, for what she went through and many other reasons.
 
Starlight Glimmer never had any of that. The general consensus seem to prefer Sunset’s character over Starlight for many various reasons and by quite a large margin. Not everyone has to agree, but its not hard to see why. (shrug)

@Background Pony #C35A  
But certainly not as many as she did after RR, as far as I remember. Back then she had her fans sure, but she was almost as divisive as Flash was in their debut.

@CronoM  
Great points.
 
Starlight panics about how ponies will see her, but most have no idea what she did. They had a problem with Trixie, which isn’t surprising after what Trixie did to Ponyville. The ponies who did remember, the ones from Equaltown, they cheerfully welcomed her back after that initial apology. Her ideology is forgotten in favour of showcasing her ludicrous power.
 
And, in a way, Sunset’s path can be explained somewhat from how Celestia and Twilight interact. Twilight freaks out over tests; comically so since she’s a genius and aces nearly all of them. On the other hand, Celestia has been shown to test her by expecting her to do the opposite of what she told her to do, or not telling her everything. So, perhaps Sunset Shimmer got sick of these ‘games’ at some point and decided she would be quicker at her own pace.
 
@Beau Skunky  
That last sentence is very important! There’s a reason “Earn Your Happy Ending” is a thing on TvTropes.

@Beau Skunky  
Exactly, they actually made an effort to not have Sunset come across as instantly redeemed in the eyes of everyone. She was despised or at least not trusted by everyone, including the Humane Five and Twilight, until the very end of Rainbow Rocks. Starlight, though, immediately got both that and a personality transplant in a montage. Nobody holds anything Starlight did against them.

@Beau Skunky  
Yeah, even her first appearance in EQG1 may not be great, but she was at least still interesting.

@Beau Skunky  
You would be laughably wrong to imply that Sunset didn’t have fans before Rainbow Rocks

I remember a time when Sunset Shimmer used to be as hated as Glimmer or Flash Sentry. So it’s rather ironic people are getting so defensive of her now honestly. In fact, honestly, I feel Sunset’s heel-face-turn/redemption & personality is just as shallow as Glimmer’s sometimes. (Though, personally, I don’t really hate either character.) Granted Sunset had to suffer a bit for her’s, and they were so successful at woobie-fying her in “Rainbow Rocks” that now everyone loves her. Apparently, it didn’t work for Glimmer though, as she didn’t have to go through as much.

@Hateot  
The wording of it in episode made it sound like Sunburst was one of the many students she was aware of in her school, but not even close to her personal student.
 
 
@Spinny  
To elaborate on @Holofan4life, she has significance and connection to the main story through Celestia and, throughout the story of EG1 and EG2, we are given an infinitely better picture of what that was compared to Starlight. Heck it even explains how in the main show why Celestia keeps Twilight at arms length, never really confiding in Twi outside the odd personal joke or opinion.
 
We know Celestia. We know how raising a little sister turned out. We know the expectations she gives Twilight has turned Twilight into a brown-nosing addict. But people deal with pressure in different ways. Twilight has low-self dependency but has had an overall perfect life. Sunset mentioned right off the bat that she knew things about Celestia nobody knew, meaning Celestia didn’t keep Sunset at arms length, but the way Sunset talked in EG2’s night scene, the pressure was still there.
 
Instead of becoming a brown-nosing addict, Sunset believed she knew better then anybody else because she was, princess-goddess of all Equestria, Celestia’s personal student AND prodigy and thus believed she knew better then anybody else. Not an illogical conclusion for an incredibly smart and clever teenager. Celestia’s high expectation teaching, even with a slightly hardy ego, could easily turn one hyper ambitious in one’s rebellious teenage years. Its believable and it fits.
 
Yet, furthering the argument even more, she wasn’t completely evil from day one. While she did have ambitious intentions, she did show Flash her sensitive side once in a while while they were still dating, as revealed in EG4, but she eventually drove him away as her ambitions neared their peak, and near the end of EG1 when she had Spike hostage, she gave up on the easiest path because she couldn’t stomach it. Then we saw he crying as she put on the crown and the worst parts of her personality amplified horrified her and put the fear of Faust in her, and you know the result.
 
In EG2, she isn’t wiped clean. She’s strong willed but she’s uncertain, she’s reflecting a lot, working on people’s resentment of her, and working on her own anger while still trying to be a good person. Its a clear struggle and it fits so well.
 
…
 
With Starlight, what is there to say? Infinitely less of her old personality remains in S6, she’s not nearly as burdened by her mistakes and barely anyone mentions it beyond it being considered a joke, was given a time skip of all her development with the Mane 6, so for all intentions and purposes, she becomes a mix between OC and a Self insert character’, an ‘original insert character’, put into the heart of the story be being Twilight’s apprentice with NO natural progression into that possession. Her mixed ideology is forgotten completely, even her actions are labeled as non-nonsensical progressions by Spike in the Christmas story episode. In the actual episodes her progression is mocked.
 
Even the Sirens pointed out that the Humane 6 still had issues with Sunset from the past, they didn’t put her into their band initially, and when Sunset tackled Rainbow, they had their doubts. It was extremely real.
 
And Starlight’s backstory? Not only is it completely removed from the main story and the initial backstory made little sense and needed elaboration, the elaboration they DID give in S6 made sure her backstory made NO sense. She already had more magical power then Sunburst did before she got her cutie mark, meaning she could of joined Celestia’s school at any time.
 
The mundane nature of her backstory doesn’t make her path to villainy more real ‘like some of real life’s greatest villains’, it simply makes it seem rushed, especially when her backstory makes us believe there were easy alternatives, since she could of joined Celestia’s school the same time Sunburst did.
 
 
@Spinny  
@Crazieman  
The communication book, in lack of a communication dragon, and the way Sunset talked about her pretty much sealed away all beliefs about her not being Celestia’s personal student for most.

(giltched repost)

By this time next year, we’ll have probably seen the advent of Glimmercorn. And we still won’t have gotten even a mention in the show of Sunset actually existing, nor will Sunset have gone back to Equestria briefly during the Equestria Girls specials and not expressed any intent to ever do so (never mind that her whole arc started by running away, which she’s still not faced up to yet).
 
Just saving everyone the inevitable disappointment now. Never mind that Twilight confirmed all the way back in “Equestria Games” that the Equestria Girls series is non-canon, which tends to get ignored for whatever reason. That’s not to say they won’t retcon that in the future, but it’s unlikely at this point.

@Starswirl  
In the first movie I think Sunset also mentions Celestia replacing her old position by taking Twilight under her wing.

@Spinny  
“As flimsy as Starlight’s reasons were for becoming an evil cult leader, at least we actually got to see it.
 
As for Sunset, everything about her past is only implied. In fact, there’s barely any reason to believe that she was ever a good person.”
 
YES, that’s what I’m wondering ALL THE TIME too. Sunset is my most favorite character (since her redemption), BUT I will never be satisfied with her backstory. And I will never believe she was born evil and was never good (she’s a regular pony, for Celestia’s sake)… I made up my own headcanon for what started her malice, though, but it’s useless anyway.  
Starlight is my second favorite, but her backstory at least has its rootage. And unlike everyone else, I for one can see her reasons logic since I can relate to her a lot.

@Crazieman
 
@Spinny  
It was very heavily implied, though. Sunset compares herself to Twilight multiple times, refers to how she studied under Celestia directly, has a book to contact the princess, and Celestia mentions she hoped Sunset Shimmer would come back to seek her guidance. To be fair, Celestia also calls Sunburst “an old student of mine”.

@Crazieman
 
Yeah, so the idea is that Twilight was Celestia’s personal protege, but Trixie and Sunburst attended the School for Gifted Unicorns.

@PonyPon  
They were both simply “students”.  
Sunset was never said to be “personal” in any way, outside of the comics. Ever.

@Helios the Dragon  
damn right

@Hateot  
All that’s said is that he went to the school.
 
Celestia makes a statement some time afterwards that implies she at the very least remembers him and what he does.

@Hateot  
Ambiguity. I hope it is a blanket reference to students at her school.  
Unfortunately the wording is almost identical in tone and context to the first Equestria Girls introducting Sunset.

@northern haste  
That only shows that they went to the same school as Moondancer and Twilight’s other friends but as far as we know none of them were Celestia’s personal students like Twilight was. Sunburst, however may have actually been her personal student once. I can’t remember the specific wording she used, but it sounded a bit unclear on this matter at the time.

@northern haste  
Okay, lets clarify, personal student.

@Spinny  
Not a personal student like Sun or Twi, but she allegedly attended the school.

@Holofan4life  
She was in the same magical school as Twilight…least that’s what the books tell us

@Crazieman  

@Crazieman
 
I thought Faust said she was.

@Spinny  
No, Trixie was never a student of Celestia’s.