I feel like this anime was just about perfect. My only regret is it wasn't longer.
I actually watched it twice, After i finished, 2 days later i started it over again.
What makes the story a 10:
The plot thickens just about every episode.
They address everything that makes the world different. You know the world in which they live is small, You learn every bit of it.
What makes the Animation a 10:
I don't know exactly, I'm not an animator, just a fan. I LOVED that animation though, The hair/sky/scenery was simply beautiful.
What makes the sound a 10:
There's nothing particularly amazing about the sound, it's all done correctly, and they didn't add annoying music at innappropriate times, the sound quality is also fine.
What makes the characters a 10:
I felt personal with all of them, and sad when they faded away. That means they triggered my emotions, and that's what anime is all about.
Why the whole thing a 10:
The "ending" was sad, but the "True ending" started to feed us ideas about what was going to happen, the anime as a whole was gratifying and it all made sence. The plot twist at the end had me amazed. As well as the story/animation/sound/characters all being perfect.
I feel like it was a perfectly executed anime. I absolutely adored it. In my opinion it is the best anime i have ever seen.