The characters, with the exception of Nyu, are all well done and carry with them a sense of purpose in context of the show. Another thing about her that irks me so strongly is that she was unnecessary; Lucy is not without her good sides, and for them to use such a weak plot device (split personality induced by blunt trauma of all things; would have been more believable if the mental pressure of experimentation and captivity had caused it to happen) to create Nyu... ah, enough, I could write an essay on how comedic characters ruined many great anime. But, even with all that, the bravery and maturity the series showed with the rest of the character developments lifts it out of the mediocre, and infuses the experience with legitimacy. It would have felt cheap if the plotlines for the characters were laced with "shock" devices, but it felt more genuine than forced, and is a major plus.
The production values are also quite good, even if it seemed inconsistent in some places. The opening, a haunting semi-replica of Gustav Klimt's more famous works with the background music of a seasoned soprano, is terrific and unforgettable; easily one of the more distinct openings of all time. The characters are well drawn, and although there is an overabundance of fanservice (thanks to you-know-who!), it is counterbalanced by the overabundance of child violence, always delicious.
All in all, this is a mature and dark series that kids should avoid. Although the childish/comedy aspect of the series and the short length keeps it from being an timeless classic, the strengths of the plot and characters makes Elfen Lied an unforgettable anime.