It's hard for me to be objective about AIR because I loved it so much. This is the only anime where I find the main character to be truly enjoyable (besides Now and Then, Here and There). He is not a hero, but he is not a lazy slob, or a villian, or an innoncent high school boy - or any other random anime cliche you can think of. He is literally a normal, sarcastic guy; someone you wouldn't mind having over on a Sunday evening. I believe that is a good chunk of this anime's charm.
I enjoyed the plot of 'The Girl in the Sky' journey - searching for a girl with wings who is always alone. None of this anime will make sense until episode 7 when the anime goes into an arc about Feudal Japan and the original 'winged people'. After this arc, when it returns to present day, I was able to enjoy this anime so much more because I understood. And the way it describes the souls of winged peopole being put into human bodies was heartbreaking.
The characters, in my opinion, were phenomenal. Mizusu, the main female lead, can become irritating to some, but if you look past that and see her pain and hurt, it's much easier to love her. Yukito, the male lead, is full of sarcastic come backs and is just a typical, 19 year old boy. The two mains are also backed by a group of wonderful side characters as well, including one of my personal favorite characters of all time, Kano: an eccentric, high school girl who says the weirdest things.
As you can see, I am already doting over this anime, so maybe this review is too subjective to be taken seriously. However, I can say this: the interpersonal relationships between the characters, the animation itself and the plotline is enough to draw anyone into it. Somehow this anime can find your soft spot and hit it, hard. You may not cry, but you will be moved.