In the film version, why is Jessie so optimistic that the new owners will treat the farm better? Because she's seen firsthand that animals (namely pigs) are capable of great evil. Though that would normally leave one cynical, it also taught a strangely optimistic lesson to Jessie that if animals are capable of great evil, then perhaps humans are also capable of great good.
The film version features a happy ending with Napoleon's regime collapsing. While this might seem a bit cheesy and shoehorned, Napoleon's reign crumbling isn't too farfetched: Jones lost his farm due to his vices. With Napoleon adopting some of Jones' worse habits, like alcoholism, it is possible that those same vices destroyed Napoleon's health and administrative prowess causing the farm to collapse as well.
It also at least comes with some degree of Real Life Writes the Plot. Napoleon's regime was based on Joseph Stalin's control over the Soviet Union. Stalin died prior to the 1955 film, excusing Napoleon's overthrow there, while the Soviet Union had collapsed by the time of the 1999 film. Combining this with Napoleon's exagerrated incompetence and self-indulgence in the film, and likely had nothing to thrive off of with the other animals gone, it can be assumed that the pig unwittingly paved his own ruin.
The book predates Nineteen Eighty-Four by a few years, but in Animal Farm, you can find several themes of the later book. Namely the power of language and propaganda to manipulate others.
The first time language is used manipulatively is when the chickens protest that the "four legs" rule would exclude them because they only have two. Snowball uses a weird argument to claim that wings can count as legs. The chickens don't understand the complexities of the argument but it sounds nice to them that they stop complaining.
Napoleon and Squealer slowly rewrite Animalism's rules for their ends. Despite violating the spirit of Animalism, Napoleon and Squealer get away with their crimes because they manipulate the letter of the law for themselves.
Squealer's constant denunciation of Snowball works well enough that the animals are brainwashed into believing he was on the side of man all along.
Censoring "Beasts of England," the song central to the revolution, with a song that celebrates Napoleon, also a means by which Napoleon can manipulate the animals into worshipping him even as he betrays them.
In the 1954 film, the pigs are the only animals who speak for the most part while the other animals use normal animal sounds to communicate. This actually works well as a metaphor for how the pigs are becoming more like the humans.
Snowball is nowhere near as bad as Napoleon and Squealer, but he's far from a hero; he participates in the earliest instances of the pigs reaping some of the benefits of the revolution while the other animals don't get to share in them. However, from what we can tell, he really does believe in the dream of Animal Farm and wants to actually improve things; while things were on shaky ground before, it's only after he's driven off that things really go to Hell in a handcart, and the pigs start violently oppressing the other animals. In other words, the bad behavior of the pigs... snowballed.
It could also be a reference to the term "snowball effect" as he's the one advocating for the animals to spread their rebellion to other farms, similar to how western nations feared the spread of communism around the world.
Pigs are the only animals on the farm that have to die to be useful. Benjamin and Boxer and Clover, they pull carts and work hard. Geese and chickens are useful for their eggs (and there is an early fight for their rights to keep some of their eggs and raise the next generation). Gander, male goose who is not as useful on the farm as female geese are, is the first character to die as a victim of the system because of his failure to build the windmill on time. Sheep, cows, goats, all produce milk, with sheep producing wool as well. Even cat is catching mice, sparrows and pigeons, which can be understood as guarding the food supply. Dogs protect the farm. Only pigs neither produce anything nor do any useful work.
Clover's reaction at the end. Orwell didn't say her reaction but we can either assume she's either horrified, completely broken, or both, with all that's happened.
Considering what the Pigs did to Boxer, what's going to happen to Clover now that she's become infirm and partially blind.
While Mollie enjoys a decent life at another farm, what will happen to her once she is inevitably worn out?
