Executive Meddling: According to Okamoto Lynn's official homepage, the reason Nozomi was written out of the anime was because the director of the anime hated her and thought that "it doesn't make a difference whether she's there or not", despite her being one of the main characters... and singing the song/poem by Eduard Mörike that actually gave the show its name.
Fan Nickname: For reference, two names are not official. "The Unknown Man" is the name given by fans since he never gives his name nor anyone else does in the three chapters that he's featured. Also, the term "diclonii" is just the catch-all term given by fans since it is strange to say "dicloniuses" (only "diclonius" has been given officially) and it is the common suffix change, so the name mutated.
The "stone stairs", where some of the most emotional scenes take place, lead to the Buddhist Gokurakuji temple in Kamakura. The upper parts of the stairs are part of the temple itself and get closed off at 5 pm every day, so it would not be that easy for the characters to enjoy the view after sunset. In both the manga and anime the gates that block access to the upper part of the stairs are conveniently left out.
Actually, the whole series is set in a real life location, Kamakura. It's very eerie to see the same locations (the dirty beach, the stone stairs, the various shrines and the island with the tower).
Kaede means Maple. Guess where the main character lives...
The entire series makes reference to El Greco's paintings. In almost every episode, characters make a hand sign, which is repeated also in El Greco's paintings.
The handsign mentioned above is a variation on a Hebrew handsign. The whole story is played out like a holocaust as well, with the scientists (nazi-like figures) not only exterminating Diclonii, but also doing cruel tests on them
Lucy is the "mother of all diclonius" according to many characters in the series. She is the only one anyone can find that can breed, and therefore she is the first known ancestor of all of the next generations. She is named after the Australopithecus fossil, Lucy, oldest known human ancestor (At the time): http://en.wikipedia.org/wiki/Lucy_%28Australopithecus%29
Nana is 7 in Japanese, which was her room number in the labs and her age at the start of the series.
Elfen Lied is German for "Elf Song". The manga ends with "Das Ende", meaning "The End" in German.
The English Language Voice Actor for Mayu's mother is Christine Auten, who also does the voice of young Kouta in flashbacks. The Japanese Seiyu for the mother character is not listed in any source than can be found, perhaps owing to how hateful and hated the character is.