For the most part, the play is written in straightforward, realistic dialogue. The characters talk with the speech patterns of their day. O'Neill, however, doesn't seem to be satisfied with mimicking every day speech. This leads to a good amount of poetry in the play. Besides the numerous direct quotes from other poets, O'Neill infuses much of the dialogue with a poetic sense. Edmund's speeches in Act IV are probably the best example. Check out his "Character Analysis" for a taste.
O'Neill's own poetic voice seems to come out most consistently in his stage directions. He's incredibly lucid and accurate in his descriptions of facial expressions and physical gestures. Check out this action: "[Mary's] face lights up with a charming, shy embarrassment. Suddenly and startlingly one sees in her face the girl she had once been, not a ghost of the dead, but still a living part of her" (1.1.92). Pretty good, Eugene.
It's almost a shame that some of the most impressive writing in the script is never heard by a live audience. O'Neill, of course, could never have reasonably expected an actor pull off exactly what he describes in his stage directions. The reason he bothers to be so descriptive is for the sake of people reading the play. In O'Neill's day there was very little legitimate theatre in America outside of Broadway. The only way most Americans got to experience a new play was through reading a script. Play scripts were once big-time best sellers. Given that information, it makes a lot of sense why O'Neill would take the time to craft such masterful descriptions in his stage directions.