I like the animation style and I love the characters.
But, the story....where do I begin?
I mean, the story could have been A LOT better than it already was, for one thing, tone down the tension such as Canterlot being occupied for starters. Maybe have a badly injured hippogriff that managed to escape the Storm King's forces race into Canterlot and reveal what has happened. To this end, the Princesses halt the Friendship Festival to investigate what is going on in Hippogriffia while Twilight and her friends go find additional help from other nearby kingdoms. They then head to Hippogriffia to help the Hippogriffs with their kingdom being the one that is occupied by the Storm King's forces.
The Storm King would still get the princesses' magic by using the hippogriffs as a bargaining chip or something like that, I don't know. It seems half baked at best.
Overall, I'd keep the characters and the animation, but CHANGE THE STORYLINE!!