I don't mind this episode it wasn't my favorite episode, I agree it was rushed and should have been split into two episodes and if anything saved for the end of the series and generally could have been handled better. That doesn't mean I have an issue with Twilight becoming a princess or that I now hate the episode, the show or the innocent character; Twilight becoming an Alicorn makes a good deal of sense on paper and actually works fairly well with what's been seen (was initially planned) and foreshadowed.
Honestly I find it a bit weird, I don't recall this kind of fuss being raised when the mind raping, innocent torturing, world destroying villain got "redeemed" after abusing Fluttershy for twenty odd minutes; but Twilight gets a set of wings and a cool title and everyone suddenly declares the show ruined.
Honestly I think lots of people are upset because the show is different now and the status quo changing is too much for some fans to handle. I feel Lrrr (ruler of the planet Omicron Persei 8) can best sum up why people some people are upset "we fell in love with you when you were this way and you changing scares and confuses us!" or the Nostalgia Critic "I don't want change I don't want change everything has to stay the same!" I do not think this is a good reason to be upset, some shows are purely episodic; The Simpsons for instance, continuity is not a thing there. MLP is it has a subtle continuity but it's there, so characters changing developing gaining new friends and moving up in the world is quite natural and it seems unfair to punish someone for succeeding and not being who "we" expected them to be.
Another reason I think for all the hate is because people om average dislike success and ambition. The TV Trope "Ambition is evil" discusses this, where a character gains ambition or succeed at some goal but loses everything and then sacrifices their success because it made them a bad person or lose contact with those close to them. The problem with this is it basically means that "succeeding is evil and you sell out by doing it" I could speculate on why people dislike seeing character they love move up in the world and become success's but I imagine i'd just anger more people by doing so, but I still think that these are two of the reasons for the hate; people don't like the familiar being changed and they dislike characters moving up in the world.
Some have expressed disdain for the musical numbers, primarily because they feel it messed up the plot or because it was to kiddy or something to that affect. Whether you like the musical numbers is personal opinion, but the writers only had one episode to do this episode and it would have been harder to tie the plot together without the musical numbers so it was a practicality choice. As to whether they were kiddy; well to be honest so have pretty much all the other songs, it's a show for little girls, I think it's just the surrounding bitterness towards the episode making them so reviled. regardless The writers were initially told they only had 13 eps and that it was the final seasons so they tried to wrap up the plot then and there. But then they got renewed but didn't get more episodes and had to stick with the ones they'd started on; given more time they probably could/would have put it into two episodes but couldn't, that isn't their fault.
As for the Cutie Marks and destiny controversy; this is just personal opinion but I'm not sure that the marks fixing plot had anything to do with destiny. Marks are still just a pony's special talent, Twi didn't just switch their marks, their personal history's changed with it and they tried to live someone else's life and needed a friend to help them back to what they truly loved doing. It seemed mroe like a final test to see if Twilight could master a magic even a great unicorn like StarSwhirl couldn't (friendship) and to see if Twilight could handle a problem like this and help her friends and guide them in a time of confusion.
Some have been saying that Twilight becoming a princess is a bad lesson for children and doesn't make sense considering how royalty works; this argument confuses me. In regard to becoming a princess, this is a fantasy world inhabited by pony's, why woudl they have the same rule's for royalty we do? There was never anything established in canon saying that you had to marry royalty to become royalty and that commoners could never rise to the position; heacanons being jossed is annoying (trust me I know) but hating Twilight for it seems unfair especially the level if bile I have seen. Beside I think a much worse message to teach children would be that if they want to be royalty, powerful, respected or successful then they'd have to marry into it as opposed to earn it. Teaching kids they can become great through being good to one another seems like a very good lesson and comparing/making someone who is loyal, generous, kind, brings happines and is honest to royalty seems like a great thing to teach kids.
There is the immortality/leaving her friends debate. Some feel she will have to leave her friends behind now that she's royalty; ignoring the ambition stuff form earlier i'm not really sure why people think this. Cadence was a foal sitter for years and married either a noble pony or a commoner, I don't see any reason Twilight couldn't simply continue her magical research in her happy Ponyville lab as always.
Also I don't think it was established whether or not Twilight is immortal; she certainly might be, but we don't know. I'm actually someone who doesn't think of immortality as a curse, it sounds awesome for the most part and there's a comic (with dinosaurs!) that help summarize why I think immortality is so loathed in media here's a quote (You know in those stories there's this immortal guy and they talk about how bored they are and how boring life is after 5000 years or whatever? I am going to call something. I am going to call SHENANIGANS.) It's we mortals that write stories, we live, we age, we die. How could we understand the mind of an immortal and why do we assume it's a nightmare? Maybe it's jealousy? the TV Tropes page "Living Forever is Awesome" is probably more the MLP definition. An eternity to help others, to learn more and to make friends, remember the old but never grow cold because there's a million new things to see and do and billions more wonderful new friends just waiting to be introduced to "you".
Another reason people have for disliking the episode is because it's a "marketing ploy to make money" I feel the need to point out that, that is exactly what the entire show has been, yes it has good writer, good characters and a great setting. But it's still owned by a corporation that wants to make money, and right form the start they had it re-made to make money, the characters were based around being sold as toys, this is not something new this has been happening the entire time the show has been on the air. I'm not sure why the writers and the innocent character have to be hated for stuff beyond their control and that isn't that different from before.
I feel the episode keeps true to the tone and messages of the previous episodes and seasons, friendship and being a decent person/pony is just as important, the strength of friendship is still shown. If the issue is the episode focusing on Twilight, then I have this to say: One of the main things that attracted me to the series was that it did well with an ensemble cast in ways that many other shows fail to do, but I know and have known since the beginning that Twilight is still the central protagonist, she always has been from the start. Her becoming a princess and doesn't cheapen her friendships or experiences, not to me at least and she's not even in my top three of favorite characters. Though she is in the top ten.
Anyway those are some of my thoughts; i'm not saying people can't dislike an episode, I dislike a fair few, but I thought i'd discuss why I think some people dislike and simply put I don't think any of these reasons are enough to declare the series over with. If you feel differently that's fine, in the same vein if you do hate the episode and the series/character now; why hang around? If I stop liking something I don't constantly subject myself to it and try to make others see it, I just leave, it's easier and healthier for me and everyone else as well.
My apologies if I offended anyone with this post, I did not intend it to be rude or inflammatory, I'm just making it to get some thoughts in order and put some other ideas out there.