I agree with you completely. Tempest is an interesting character (I guess) in certain terms. And I read the prequel stories and all that, and I still dont' find her nearly as interesting or as complex as Celestia and Luna. Their situation was a lot more emotionally charged and painful. As for Tempest, little ponies get picked on all the time. The story also really lacks actual information on her parents. Didn't they support her, and love her? Take care of her after the accident? I understand her being upset at her friends and not being too trustworthy of making new friends but as for having people that loved and cared for her---shouldn't her parents have had a hoof in that? And didn't it mention she left the town at a decently young age? HOW? This comes back to the parents thing. Did she NOT have any family that gave a flying pegasus butt about her?
Tempest and her reasons for becoming "evil" were shallow and underdeveloped. Not to mention, the Storm King just seemed like a humorous foil. Like, he wasn't the "real" villain. Tempest was supposed to be, and she wasn't really all that threatening. Tirek and Discord were more threatening. Especially Tirek. Even the Prequel stories told little about the Storm King. Why should I be intimidated by him? He did nothing but just show he was the cookie cutter villan that wants all the magic and all the power, with a 'humorous edge'. It's not funny, it's pathetic. He was laughable. I've seen numerous 'blind watches' on YouTube about the movie and a decent bit of them believe that the Storm King was just a distraction and again, laughable.