Most things have already been said, so I shall keep it focused:
Some people may complain about the ending, about how the series lost its momentum in comparison to the one-shot; however, I think the final development is very befitting. Things mellow out as you grow older. And as much as Koe no Katachi is about bullying, it is also a coming-of-age story.
Events and people that seemed so crass, so insufferable in your youth become bearable as you learn to put them into perspective. Even more so as time makes you realise that people cannot change that easily - and they don't have to.
This does not mean that one should just forgive and forget. However, life will never go exactly the way you want. Not everybody can get along with each other, in the same way that you cannot like everybody; not even among your close acquaintances - but that's all right; we're only human after all.
Once you've accepted this fact, you'll be able to take life's various unpleasantries in good humour,
just like Nishimiya Shōko does in the end.
What really counts is to grow aware of one's own flaws and attitudes, so that when the time comes and you want to change, you'll know where to begin. Or to say it with Ishida Shōya's words
"I think you're fine the way you are [...]. I think you'll be able to change when you really need to. What you like and what you don't isn't everything".
I believe that is the message Ōima Yoshitoki-sensei wanted to convey when she chose to leave some conflicts unresolved.
Same goes for the romance. Don't be bothered by the "Unresolved Romance" tag! Some people might see it that way, I personally don't - but does it matter? The main "romance" - if that's what you want to call it - is realistically subtle, and no matter how it goes, life will go on for our protagonists. We know that they'll be fine regardless of what life may throw at them because they have matured throughout the series and will continue to do so. That's all we need to know as readers of this marvellous manga.