There's way too many different things that make a song good, too many to articulate. Such as musical talent, love for the band, lyrics, meaning, uniqueness, emotion... etc etc.
If it had to be just one single thing I would have to say emotion. Nothing bugs me more than emotionless instrumentals or over-produced radio garbage. When a band puts out a song to show off technical skill it just makes me want to cut myself. I'll take Gilmour over Satriani any day.