As a note, I read the manga, but that was years ago. I know the anime differs greatly from the manga, but I like the additions because they ease a bit of confusion that the manga presented.
Story: The story mainly focuses on Kunisaki Yukito and a girl he meets at the beach, Misuzu Kamio. However, the story also moves, almost episodically, around the other women of the small beach town, all of whom seem to have ties to the "girl in the sky". The slow pace and quiet setting are dramatically foiled by the brief glimpse of the past, giving the origins of the legends that center in the anime.
Animation: Kyoto Animation tends to put this anime up as one of their best. And with good reason. The beautiful backgrounds and character designs make this anime visually stunning.
Sound: What more could a dramatic anime wish for than this soundtrack. Short of using Vivaldi's "Summer" orchestra set, this anime audibly brings out all that makes a lazy summer afternoon. The music sets off the story, enchancing audience reaction and definitely increasing the appeal of this anime.
Characters: The characters' individual storylines might give a little insight to the legend of the "girl in the sky", but overall, they take away from the main plot surrounding Misuzu Kamio. The characters are all a bit underdeveloped due to their brief, intermittent appearances. While the side-character designs are beautiful, they are essentially filler to make the anime longer. This filler status limits the side-characters to arbitrary, stagnant roles.
Overall, I liked Air. It's sad tale is enjoyable and beautifully done.