Mourning Becomes Electra takes many of its plot elements from the Oresteia by Aeschylus, including the mother of the family taking a lover while the father is at war, killing her husband, and in turn being killed (directly or indirectly) by her children. Despite these plot elements, O'Neill's play is distinctly modern. It is a psychological drama rather than a classical one.
First, although the characters are from a wealthy family, they are not royalty and thus the play is about individual people rather than the broader civic implications of the original trilogy. It is a psychological drama focused on human feelings and motivations and lacks Aeschylus's concern with the gods and fate. Additionally, justice is more personal than universal, despite O'Neill's effort to introduce more general themes.
In staging, although O'Neill wants the acting style to reference ancient tragedy, the play is still basically modern. Women are played by female actors. Actors do not wear masks. The play is written in prose rather than verse, and Seth, although serving a function similar to the chorus and even singing briefly, is not the equivalent of a full Greek chorus, which sings and dances for long periods between episodes.