Total Recommendations: 4
The first five minutes of the very first episode of Xam'd gave me an overwhelming Nausicaa vibe, and the feeling really hasn't diminished with successive episodes. I guess the most obvious similarity is that both feature a heroine who flies a single-person light aircraft and grows things in jars and is tough yet deeply compassionate, to the point of defending or weeping over creatures others would think of as monsters. Also, there's lots of airships and goopy stuff and morally complex war stuff and a cute animal. Basically, if you like Nausicaa, be sure to check out Xam'd. If you like Xam'd and you've never seen Nausicaa... have you been living under a rock? It's a classic!
In both these series, the ultimate conflict involves an emotionally damaged girl having to come to terms with her past trauma through the help and friendship of another girl. The latter parts of both series have a similar mood.
In a lot of ways, Oniisama e feels like a spiritual predecessor to Utena. You have a classy private school setting, a large cast of primary, secondary, and tertiary characters, dark secrets, complex motivations, twisted relationships, and a unique atmosphere created by combining lush shoujo imagery and visual symbolism with music. Oh yeah, and lesbians.
Both Utena and Tutu contain a strong fairy tale motif and themes of accepting or defying one's fairy tale role. In a way, Utena feels like a more sophisticated/jaded exploration of the same themes touched upon in Tutu. They also both have a little bit of that surreal/reality-bending element, though Tutu never gets quite as surreal or symbolic as Utena. Basically, they're both really great series that you should watch, period.