Not only that, but one of the romantic relationships started off with two people simply texting each other on cellphones and how one's Start of Darkness began with them falling to their death while they're on their cellphone.
Near the end, a song called "Eyes Glazing Over" was used as the ending song. If translated, one of the lines is "Don't ignore me, listen." As in "don't ignore me and listen to what I have to say", which makes sense when, by the end, everyone starts to understand better.
Keitaro sneezes in one of the early episodes, and claims that it's his allergy to dirt. Every other time he sneezes, it's in the presence of an Orphenoch - and it seems that's what he's allergic to. Now, when he sneezed for that first time, the only people nearby were Mari and Takumi ....who is later revealed to be an Orphenoch.
Early on in the series, Takumi does not seem to be phased, or surprised by the existence of Orphenochs. At first it seems it's just because of his "I don't care" attitude but later it makes more sense - since he is an Orphenoch himself, it's only natural that he was not surprised.