As someone who hasn't read the manga, I'm confused as to why some people rated the movie so low. The manga must have been a masterpiece, huh. Why can't people just review it as a standalone movie?
Story - 10/10
Rarely have I ever seen a movie/anime/book where the main character bullies someone and gets bullied back, effectively isolating him from his classmates. Do you know why? Because that doesn't happen in real life. Bullies become popular in high school and have a huge follower group. Despite this, I was glad that Koe no Katachi took a different route and made the bad guy into an even more bad guy, then switched him around and made him a good guy. The plot was fresh, in my opinion, and I loved (well love isn't the right word, maybe "invested in", or "hooked on" would be better) every hill and turn of the rollercoaster.
Art - 10/10
Bright, vivid, and set the mood really well. Nothing less than expected from KyoAni.
Sound - 9/10
In the moment, the songs were great. The OP was enjoyable, though I was a little confused about what was going on on-screen. Anyways, that's not sound. The only thing stopping this from being a 10 is the fact that I'm not humming any of the tunes right now. Maybe that's just my harsh criteria.
Character - 9/10
I have to agree with most negative(ish) reviewers on this one. Aside from Mr. Main Character and Ms. Main Heroine, the side characters, save from Yuzuha, didn't get much development. I was a little perturbed as to what each of their motives were. For example (not in the movie, so not a spoiler), I didn't know that Nishimiya's mother caught a virus during childbirth, resulting in her daughter being born deaf and her husband leaving her. I only learned that by reading the character description page on MAL. That, and other information, would have enhanced the movie as a whole. The little side stories is what Koe no Katach is lacking.
Enjoyment - 10/10
I was *this* close to crying, and I'm disappointed that I didn't. This movie deserved it.
Overall - 10/10
Being a high school student myself, I feel that Koe no Katachi struck home with me, having dealt with bullies and may have been a mild one in elementary school. It really puts into perspective how important it is to have respect for others and not be a total douchebag. Or else you'll end up like Ueno, a distasteful, cold-hearted, jealous baka. I seriously hate that broad.