Instead of "Final Fantasy", it should've been called "Final Post-Apocalyptic Science Fiction" (let's hope it's the final one of those, too), because that's all it was, except for some downplayed spiritual overtones which were lost on many audiences.

Virtually no 'fantasy' in it. Unoriginal plot, unoriginal soundtrack, unoriginal movie in virtually every way, relying on the animation to sell tickets.

The FF games demonstrate that Sakaguchi can write and direct some very original stuff. Shame he couldn't be bothered doing so for the big screen.

I think I know the biggest problem, though. With TSW, they'd planned to create a realistic simulation of lifelike characters. Something original, different from other movies. However, during production, Sakaguchi kept dogging the animators, insisting that the characters and scenes look like they're from a 'real movie' - not 'real life', but as artificial and airbrushed as regular cinematic fare. Instead of simulating real life, TSW simulates a simulation - the artificial worlds created by Hollywood. That's why it was doomed to critical failure. All the 'movie' traits are there - characters with perfect hair and skin (even after life-threatening calamities underground), pointless lens flare effects (mimicing an inherent fault in real-life cameras), and lacklustre directing which was made to look as though the camera was limited by gravity, pulleys, physical constraints which apply in the real world.

There was so much potential to be revolutionary, but it was squandered. An original plot, world and style would've helped to no end.
Sadly, it wasn't to be - instead we just got a computer-generated replica of something we've seen too many times.