Watching a pair of children die may be a fantastic plot if you're trying to elicit a strong emotional response, but it doesn't necessarily make for a great movie.
If you read online reviews about this film, the very first paragraph is almost always about how this film utterly devastated the reviewer and left them in an ever-widening puddle of tears. Seriously. Read the comment section of Roger Ebert's review. It practically becomes a crying competition.
Yeah, we get it. 'Grave of the Fireflies' is sad. Unfortunately, that's almost all it is.
Call me heartless, but I quickly grew bored with Setsuko's seemingly infinite capacity for sobbing and whining. Never mind Seita's horrendously bad decision-making skills. And, yes, I appreciate that he's a kid, but the fact that Seita allowed his sister to die of starvation while there was still plenty of money available to buy food just ruined the entire movie for me. You can cite Japanese cultural differences all you like, but I don't think even this can justify Seita's unwillingness to get a job, or endure a snotty selfish aunt, or put his 4-year-old sister's dietary needs before his own.