If Oreki hadn't changed, if he still followed his stated philosophy of, "If I don't have to, I won't; if I have to, I'll do it quickly," then he wouldn't have gone to Chitanda's festival at all. But he did. He became less selfish and opened himself up to new experiences.
As for the movie arc, Oreki changes, but not necessarily for the better. He's hurt by the experience, and tries to shut himself off from everyone. Not because he's lazy, but because he thinks he's useless. Episode 11.5 makes that more obvious, but it's still apparent from the material. The way he dives into the movie mystery is totally different from his apathy at the beginning of the festival.
In Chuunibyou, Yuuta's only character development is related to his relationship with Rikka, she ends the show pretty much exactly where she started it, and Isshiki and Kumin basically never changed at any point.
The only thing that Oreki developed was a hard-on. Oh wait, that was established back in the hot springs episode.
No, you're wrong. Chitanda always had power over Oreki that he can't say no to any of her requests. That has not changed. Oreki still remains lazy only driven by his hard-on for Chitanda.
I'll reserve my opinion until I watch 11.5, but Oreki did not learn anything from the movie eps. He still uses his friends as pawn even when he figured out everything himself. It's pretty evident in the festival, the teacher, and the valentines ep.
Chuunibyou is bit difficult as it's more comedy than anything, but Yuuta character developed greatly nearing the end when he realizes that the chuunibyou is something that you have to live with and shouldn't be ashamed of.
Rikka had always been trying to convince Yuuta to live with his chuunibyou, but as she fell in love, she made a mature decision to force herself into accepting what people wanted her to do.
The only thing that's wrong with Chuunibyou is that it was too short. The "lack of character development" is only felt because the other three main characters were too minor. If they had more episodes to expand their character history, it would heighten Yuuta's self-conflict with his chuunibyou and gradually make him realize that it's not a bad thing (rather than the sudden realization at the end when Rikka left).
Huh? I talked about the first story arc. Chitanda was at first portrayed as this perfect anime heroine and we find out she's been harboring all this pain from her childhood regarding her uncle. Oreki too goes from uninterested slob to going more and more out of his way to help Chitanda solve her mystery. All that is in the first story arc. You can't say that's not development.
If development means establishing characters, you're quite reaching there.