First I'll say I liked Advent Children, but it had problems so I'll reply.

First plot, I don't think there is much plot, hmmm... how to explain, it didn't have much of a plot, I can't show this too well- there just was not much of a plot. Because the last 30 minutes were fighting they had maybe 40 minutes to develop a plot. But oh yeah there were also a bunch of fights there too, giving very little time for an actual plot. It just wasn't very good.

Characters- Red 13, Barret, Yuffie, Cid, all highly developed characters.
Reno- Comedy, fought bad guys, offered Cloud a job. very developed.
Rude- Same as above.
Rufus- Had Jenova all along, talked a little with Kadaj.
Kadaj- Wanted to become one with Jenova, got kids with Geostigma together for the reunion, not a lot of character nor did it change at all during the movie.
Loz & Yazoo- You can like them but they weren't at all developed or anything, they were henchmen just important for fights.
Tifa- Thought she was OK, but as this was a movie with 30-50 minutes to develop characters they couldn't do much.
Cloud- Fought the enemy... had super powers in combat (I had to remind myself about 10 times that he can't fly, it's just jumping and the wind), didn't develop much, had two problems. One was Geostigma whether to run away or fight, he chose to fight after about 5 minutes, deep. And guilt of Aeris which was done OK, except I can't stand Aeris so it didn't work for me.
Marlene- Cared for Denzel, another highly developed character. Didn't do much else.
Denzel- Hmmm... got sick.
Everyone else- even less important.

[begin blatant sarcasm] As you can see the subtlety and depth was stunning, the plot was intricate and detailed with many interesting and forseeable twists.[end sarcasm]

I want in something an interesting story with interesting characters. This lacked that.

Considering the time and budget limit, I think Square-Enix did a very good job.
OK, that doesn't defend a thing, it still should have had better characters. My opinion though is that it didn't make sense in the form of a movie, as it had very little plot, as an action movie it was OK but for an RPG based movie it could have had decent characters and an interesting storyline.

This is all my opinion.

I did like Advent Children, and this was probably written sloppily cause I'm dead sleepy.