I don't find Cloud's depression in AC to be unreasonable or a regression of his character. I agree that it's tiresome in Kingdom Hearts and in Dissidia 012 due to what the creators have said regarding the "Cloud that the fans want." But taking the Compilation's portrayal of Cloud in a vacuum, it doesn't bother me.
On the Way to a Smile (especially the Case of Tifa and some of Denzel) essentially got rid of any doubts I had about Cloud's characterization. That story clearly displays Cloud's descent from End-of-game-Cloud to Depressed-Cloud. He starts out smiling at and with Tifa and Barret. Quietly correcting Barret's boisterous orders to the construction workers (a scene that exudes the character humor I love in FF7). Acting as a father/elder brother for both Marlene and Denzel with involvement, not detachment. It isn't until the combination of Elmyra's continuous orders of flowers to Aerith's grave, his inability to find a cure for Denzel, and finally, his own contraction of the disease that he backslides. It's a gradual progression and provides, as Octo alluded to, a lot more justification than I'm usually told that sufferers of depression have.
Remember that those comments about the Cloud that the fans want were made by Kitase and Nomura and such. Nojima writes the movie and the stories, and I think the story is more than competently told my him. Whether Kitase et. al. told Nojima to make Cloud depressed and Nojima is skilled enough to make it work, or Nojima just wrote what he wrote and Kitase et. al. are saying those things in a response to what he wrote is irrelevant and doesn't speak to the quality of the work.
The movie should have done a better job of showing this progression, and not made OTWTS required reading, yes (and ACC made an attempt, I think). It's unfortunate that the way he is in AC/C have now dominated the public perception of him as a Squall-type character when he is not. And it's also perfectly fine if you simply would have preferred to have confident/put-together Cloud in the movie. But how people suggest that it undoes Cloud's development in VII is strange to me. Especially considering, again, how easy it is to backslide into depression (I'm told, never having suffered from it) WITHOUT contracting an incurable, terminal disease as thanks for saving the world.
I've said before that if they make a post-DoC sequel and Cloud's depressed again, then I will call bullshits. But Cloud seemed perfectly fine in his small role in DC, complete with cocky sarcastic banter with Vincent, he was also perfectly pleasant in Crisis Core outside the moments that he was already established as being depressed, aforementioned scenes in OTWTAS, and even moments like his smirk while kicking Kadaj's ass. Cloud's characterization makes sense in the Compilation, and it's not AC's fault that Kitase and friends made dumb comments.
) and to atone for his supposed sins, but I'll be forever searching and waiting for the real Cloud that I experienced.