Two things that hit me first about a song is the production quality of the music and its originality.

Great music tends to have great mixing and mastering but I think a lot of the songs I come back to are solely because of the aural quality.

Originality, whether it be an unfamiliar time signature or sample / musical influence piques my ears more than a song that is well-crafted but keeps within the boundaries of the genre (if that makes sense).

But of course there are exceptions or clauses. Like I like Madlib but its not like he spends anytime polishing the track by mixing and mastering usually. And 36 Chambers sounds like a bad demo but it's done with such forethought that it sticks out to me.

As far as music v lyrics, music stands out to me more than lyrics. For instance there are lyricists like Nas who would rap circles around The Pharcyde but outside Nas' best albums I'd rather listen to Bizarre Ride II than most of Nas' catalog.