Ebert said it best: Grave of the Fireflies makes you rethink the possibilities of animated films. This isn't a pixie dust fantasy, or a heightened sc-fi anime, it's one of the most touching, poignant, and stark anti-war films ever made.
But to call Grave of the Fireflies purely an anti-war film is perhaps too simplistic (director Isao Takahata dismisses that label entirely). This film is gentle and poetic, more than it is violent, and it's remarkably restrained in its anti-war message. There's very little in the way of moralizing or polemics (probably because the story doesn't need it).
Perhaps best of all, Setsuko is one of the very, very few (if not the only) animated 4 year-olds who actually behaves like a four year old. I'm so sick of seeing preternaturally smart, sassy, sophisticated and precocious children in Hollywood movies. Setsuko's emotion and behaviors are EXACTLY right for a completely normal four year-old, and recognizing this goes a long way in cultivating a fully realized character. Similarly, Seita is a teenage boy who behaves with the sort of mixture of pride, compassion and hubris you'd expect of someone his age. He still believes that Japan will win the war. He thinks it's up to him to take care of his sister with their mother gone and father who knows where. This leads him to make mistakes: possibly the most obvious one being where he fails to take the farmer's advice, swallow his pride and ask his nasty aunt to take them back in again.
This film is pretty famous for making people cry. While I didn't have that strong of an emotional connection to it, I appreciated the beauty and honesty it displayed. This wasn't a glossy portrayal of anti-war political statements and heavy handed plot points, it was a simply about a brother and sister coping with the after effects of a city and family destroyed. The fact that Grave is a semi-autobiographical apology to Takahata's deceased sister makes it all the more powerful.