Any song (not instrumentals) has three basic elements:
music, vocals, and lyrics.

When an artist/band is hitting all of these three well, the song is great.

If any one of these elements is undeniably atrocious, the song is sunk.

Still, I might consider a song worth listening to so long as any _two_ of these elements are of good quality. For example:

Music & Vocals
In a song like this, it doesn’t matter quite as much what the singer is saying--they’re still conveying the emotion and the mood with the sound of their voice and the music.
Example: the Dave Matthews Band song “Stay (Wasting Time)

Vocals & Lyrics
If both of these elements are hitting dead-on, then I can usually overlook a weak musical accompaniement.
Example: Joan Osborne’s latest album, Righteous Love

Lyrics & Music
Common to singer/songwriters who have an unbelieveable amount of talent but, alas, were not gifted with a songbird voice.
Example: Bob Dylan, or Blues Traveler

While the given examples are biased to my musical tastes, I think it is apparent how this system can be applied to almost any musical genre.