First impressions can mean everything, in life as well as music.
As humans, our tendency is to make snap judgments, valid or not, when we are first exposed to something. That's why I can't emphasize enough the importance of having a good intro to a song. To me, the intro is usually a melodic theme over the song's verse or chorus chord structure. Here are three characteristics that I've found to be integral:
1. It has to be unique and identifiable. The listener needs to know which song they are listening to from the first measure.
2. It must be memorable. When it gets stuck in the listener's mind, they'll want to hear it over and over again.
3. It should be appropriate. To support the overall message of the song, it must be consistent with the emotion of the lyric. Also, an appropriate intro doesn't feel so repetitive that it gets annoying. Most intro themes reappear later in the song
With a good, solid intro, the listener should be prepped to enjoy and be engaged by the rest of the song.