If you liked
Air
|
...then you might like
Princess Tutu
|
I find the strongest reason to recommend an anime is the feeling you get from watching it. While Princess Tutu and Air are quite different in setting and style, I got that same feeling of really feeling for the main heroine in the struggles she goes through. I'm attributing this to the depth of empathy established in both Misuzu and Ahiru. They are both in incredibly unfortunate circumstances and have to overcome predetermined rules to succeed. So, if you liked Air or Princess Tutu because of the feeling you got from watching them, I'd recommend the other to you.
If you liked
Full Moon wo Sagashite
|
...then you might like
Princess Tutu
|
Full Moon and Princess Tutu are notably similar because of their appearance as typical Magical Girl anime, but then break the rules and present us with something darker and reflective of the genre itself. If you liked the fact that Full Moon was a children's series, but with deeper concepts, you'll like Princess Tutu.
If you liked
Air
|
...then you might like
Kanon (2006)
|
I suppose it's pretty obvious, but a lot of newer people don't realise that Air and Kanon are by the same original author; Key. Art style and some of the themes are similar, but what I find the most striking is their presentation of seasonal opposites. Air does the melancholy in summer so amazingly well, and Kanon does the melancholy in winter almost as amazingly well. So I suppose if you liked the summer feel of Air and want the winter version, Kanon (2006) would be it.
If you liked
Air
|
...then you might like
Clannad
|
While Kanon (2006) is more similar in it's Harem aspects to Clannad, Air is very thematically similar to Clannad in that it focuses very much on family. In my opinion, Air did a much better job at presenting themes of family than Clannad did so if you liked that aspect of Clannad, you'll love Air :)