To begin, the quality of The Disappearance of Haruhi Suzumiya is great. Kyoto Animation really outdid themselves this time. The backgrounds look almost real, the fluidity of the characters' animations is exceptional, and the lighting effects convey a haunting feeling of despair.
The majority of times we see the characters from The Melancholy of Haruhi Suzumiya act whacky without any sense of care. Haruhi is the crazy girl, Kyon is the centered and boring person, Mikuru is pretty much "a cutesy moe character", Yuki is the robotic and genius girl, and Koizumi is Haruhi's toy.
The Disappearance of Haruhi Suzumiya changed my notions of the characters. I still get perplexed when Kyon sees Mikuru for the first time after the world changes. More astonoshing is Mikuru's violent reaction, and I admire a lot the affection of Tsuruya toward her friend. It also breaks my heart to see Kyon suffer. But he later on shows his maturity and devotion when looking for Haruhi.
Looking at the legacy of The Disappearance of Haruhi Suzumiya, it is clear that Nagaru Tanigawa has created a very dynamic world and complex characters. It will be interesting if more of the light novels are given the film treatment. I would not mind seeing The Intrigues of Haruhi Suzumiya in theaters.