The idea would be to select the part of the song you like best with a simple time interval.
This is similar to some other ideas but this is not for playing only that part of the song, but just pointing out, that for you, this is the best part (usually the chorus).
When you are playing a song you can click the "Favorite part" button to show the indicator or not so you can add it only to the songs you like.
This information would be useful for users as they can play its favorite parts of the songs and would be useful for a new way of discovering similar and new music you may like.
That's for me the most important thing since I use spotify to discover and play new music and I feel most of the time that my favorite songs are quite similar in the chords of the important parts of the songs.
So with the information of the best part of all your songs you can look for songs that have similar chord progression but not of the whole song but of this personal best part of the song (you remove the irrelevant and useless data). Besides you can use this information to indicate a kind of average of the most voted parts of songs of all users so when you are discovering new songs (with another show/hide button) you can go straight to the best part of the song to see if you like it or not.
I think a good discovering service is very important since nowadays users like to be updated with the coolest and brand new music right away.
A similar idea has also been suggested here:
Add your kudos and comments there please! ;)