Elfen Lied doesn't really get any better. The first 10 minutes of episode 1 is probably the highlight of the whole thing, fairly dark and edgy.
After that however ... it falls into the "domestic harem" series structure (using harem in the loose sense here). e.g. it switches to a focus on the one hapless teen "everyman" male who conveniently has a big house, and zero parents, with a number of attractive females who either live in the house or who's lives orbit around it.
Sure, there are some effective scenes here and there focusing on the tragic backstories of the diclonius girls, but they don't make up for the fact that the type of structure that I outlined above is inherently a static structure. Series like this are written like that because it's easier for the writer. No matter what happens the story can "snap back" to the daily domestic life (or school clubroom) thing as a point of reference. Sure, it can work but it's a fine line between having a point of reference when needed, and the "domestic life" aspect draining all tension and sense of movement out of the plot (not an issue for some genres of course). And it's made worse in Elfen Lied because the main girl has a split personality, she has the "ruthless killer" personality and the "bumbling kawaii girl" personality, so the seams between the "action plot" and "slice of life comedy" are very demarcated. It's like two shows in one! Selling point! Basically the main story tension derives from times when the one show (horror thriller) impacts on the other show (domestic slife of life harem).