I do feel like there was a good story in the basic material, touching on Cloud's estranged relationship with his friends and makeshift family with Tifa and Marlene, as well as his lingering guilt over the death of Aerith. The big issue is that the filmmakers don't care about these things; the whole movie is about propelling us to the next action sequence and re-introducing characters you liked in the game in flashy scenes designed purely for fan service. The new characters are all bland, basically a joke. The story and characters are pushed aside, an excuse to show off the animation (which has not aged well) and have Cloud acting cool for the sake of it. Given how lazy the writing is, it is easy to tell that creating a good story was not the goal of the filmmakers.
I'd give Advent Children a little leeway if the fights were any fun, but their execution is terrible in every aspect: the editing is so rapid that it is hard to discern what's what, the faux-shaky cam making that effect worse. The fight choreography is laughable too. In the original game, at least the characters were stylized, so you could buy them doing impossible things at times. But when everyone is realistically proportioned and they are all essentially weightless, jumping miles and miles into the air holding eighty-pound weapons-- I'm sorry, my suspension of disbelief can only hold so much before it snaps. The big climactic fight between Cloud and Sephiroth at the end is supposed to be cool, but it looked so over the top that I was beside myself with laughter. Though people are bruised and stabbed, I never felt anyone was in any real danger. When someone falls off of a skyscraper and they are able to land on their feet with not even the least bit of disorientation, I'm going to stop caring quickly.
It's really hard to recommend this film to anyone outside of the FFVII fan base and even then, I'm shocked so many people there love it. It could have been so much better. It actually makes me fear for the remake, which is going to have plot alterations from what I have heard. Hopefully the end result will not be as humorless and dull as what we got back in 2005.