For those who love dark stories, Elfen Lied and Higurashi (both seasons) are great complements. Not simply because of the gore content (although none of those two animes are lacking), but also the depth of the story and the eeriness they emit.
What people would appreciate in those two animes would also be the contrast between funny and cheerful moments and the sudden ominous and serious tone that could appear unexpectedly. Characters are very likeable and can easily be connected to.
However, Higurashi is a longer series and offers more mystery and unexpected plot twists than does Elfen Lied.