I’ll be watching it through again once I’ve recovered from the first time, but meanwhile I can’t help thinking about it. Much of my speculation is sure to change as I watch again and argue with others (coughcough), but this is where I’m going with it now.

Beware! Spoilers abound.

What is Glie?

Glibly, "a boot camp for angels."

What are the Haibane?

People who had the potential to become great teachers/guides on Earth, but whose lives ended early.

What is Glie?

A safe harbor, a test, a trap. And a choice: become a true spiritual guide --- an angel --- or live a mortal life and pass into the next world.

What is the Circle of Sin?

A different test, one faced only by haibane who took their own lives on Earth. Only Reki and Rakka are known to have faced it.

What is the Day of Flight?

Graduation. The time when a haibane is forced to make the choice between there's something I need to do here and I'm ready to move forward. Those who pass become teachers and guides for others; those who fail join the Renmei (and, indirectly, guide other haibane).

What are the Renmei?

The staff of Camp Glie, so to speak. Some, perhaps all, are former haibane who couldn't manage to move forward.

What are the Nenshougumi ("Young Feathers")?

I'm not sure. Their purpose is clear: they exist to be guided by the haibane, practice souls for apprentice angels. Maybe they're the not-yet-born, maybe they're the never-born, maybe something else.

What are the humans of Glie?

I think they're the never-born, granted a complete life in a gentler world after being denied a life on Earth.

What was Rakka's crow?

A manifestation of someone's hope for Rakka's salvation, someone whose love she didn't value in her life on Earth. Parent, sibling, teacher, pet, whatever; the crow is "if only I could have reached her," and its death comes when that person believes that the message has been received.

Did Kuu help guide Rakka after her flight?

Yes, I think so. Having passed beyond, she couldn't come back inside Glie, but there was still a bond, and when Rakka was near the wall, she could make a connection.

What was Reki's mistake?

In life, ending it. In Glie, misunderstanding her test. She believed that there was only one test, the Circle of Sin, and that the "good haibane" were those who either never fell into sin or found their way out of it. In her view, everyone else would inevitably reach the Day of Flight and leave, and she was the only one who could fail and be left behind, damned. She was wrong. All the haibane must face the test of Glie, but the sin-bound must first pass their own, harsher test. She had to break free from the Circle of Sin and Glie before her time was up.

...where are the honorifics?...

Something I'm looking for when I go back through is how often honorifics are used. The only one I really noticed was the girl who treated Rakka like a good-luck charm and called her haibane-chan.

...how did the other haibane die?...

Their dreams are clearly their deaths, but I didn't pay close enough attention at the time.

...what are their true names?...

We're told that all haibane have two names, the one they take from their dream, and the true name that they discover on the Day of Flight. My guesses for Kuramori, by the way, are "dark forest" and "dream nursemaid", respectively.

...does "haibane" itself have a true name?...

Is there another reading for it that represents the ones who pass the test?

...what happens to the nenshougumi?...

They don't seem to grow up to be haibane, but they do seem to grow up and move on. The way they leave can't be through the Day of Flight, though, because Reki and Nemu had no idea what had happened to Kuramori. My current suspicion is that they simply fade from memory when they leave.

...can haibane have children?...

Highly unlikely. It's not that sort of story, though, so there's really nothing to go on. Even sex would be problematic (I'm sure the fanfic and doujinshi communities have this one covered), but they're definitely not without other passions, so how deep do they run, and how do they express them?