There are lots of reasons to like a song... great lyrics, skillful use of rhythm, beautiful harmony.. what ever your thing is.
One thing that never fails to amaze me is how music can take control of your emotions... alter moods... build surreal imagery in your mind... energize you... it's as if you experience the music in an intimate way.
Lots of people work out to upbeat music, something with a cadence or has an energy that gets your pumped up. I sometimes select music that is intentionally down-beat because I can use the "sadness" as motivation to struggle against it.
Regardless, there's no denying that music can create physical responses to it... lots of varied and different responses... but the one that makes a particular piece stand out to me is if it gives me goosebumps. Deriving physical pleasure from music is one way for something to instantly jump to the top of a playlist.
In my experience, it's something in the music, moments of transition usually, where the richness of the sound grows quickly, the mood changes, and I get swept along with it. This is one thing about "prog-rock" that grabs me because you see this kind of thing all the time.
As an example, the last 2 minutes of lateralus by Tool is my latest "whoa" moment. Another is the opening to "fade to black" by metalica... gets me every time. Classical music is good for this as well, but I generally lack the patience to sit through a whole piece to get the full effect.
What sort of music does this for you?