Don't be fooled by the angel-like appearance, by the way. Religion has nothing to do with this anime, and ABe has said before that it's a purely aesthetic choice, though some do feel that there's symbolism involved.
That said, it's provocative and heartbreaking and dreamlike. Pacing is slow, but expertly done. You'll find that one episode transitions easily into the next. And so much of the story is implied... as well as character backgrounds and the like.
Its art doesn't try to wow the viewer, and it seems content to just let the setting and soft colors and unique character designs speak for themselves. The backgrounds are gorgeous and detailed. Characters' personalities are mild and realistic; no character gets shoved into the standard archetypes you so often see in anime.
Also notable is the soundtrack. Every song fits the mood of Haibane Renmei perfectly -- especially notable (aside from the opening, "Free Bird," and ending theme "Blue Flow") are "Garasu no Yume," "Ailes Grises," "Starting of the World," and "A Little Plate's Rondo." Many of them feel like lullabies. Personally, the soundtrack is one of my favorite parts of this series, but looking at the other reviews, it looks like I'm the only one who finds it so breathtaking. Your mileage may vary.
It begins by letting the viewer into the peaceful simplicity of daily life in Glie, allowing insight into the setting and the minor characters, but it grows into a story about friendship and letting go and guilt and forgiving yourself and so much more. The climax of the story is likely to make you cry or cringe or suck in your breath -- maybe all three.
At times depressing and at times gently uplifting and feather-soft, Haibane Renmei is unparalleled in beauty, and I wouldn't hesitate to call it my favorite anime of all time.
