Add a "like" button to show your excitement for a song being on the setlist
First I thought about the option to designate your favourites among the songs you've heard live from your profile, but then I thought, why not do it directly from the setlist?
What I imagine is a button similar to Facebook's famous "like" button, with a different logo - maybe a heart, smiley or simply the setlist.fm logo - I'd go with a heart - next to each song on the setlist. When you hover over the button with your mouse, it will display "I was very excited this song was performed". Next to the button, if anyone clicked it already, there is a bigger, more noticeable heart with the number of people who pressed it inside, and when you hover over that heart, it will display the text "e.g. [name], [name], and [name] were very excited this song was performed".
1) This lets you share your excitement with fellow fans when your favourite band finally played that song you've been wanting to hear for so long.
2) It lets you see which songs other fans were most happy about
3) It could be used for artist statistics to rank their most popular/desired songs by calculating each song's average "likes" (how many "likes" it has divided by the number of times it was performed), which I think would be a really cool feature!
4) It could also be used for personal user statistics - which songs have you personally liked the most times?
I suggest you can only "be excited" about a song if you were at the concert, so clicking the button will prompt you to click "I was there" or "I wasn't there".
On a related note, there could also be a statistic for "which songs have you heard live the most times", and a "discuss this concert/setlist" chat on each setlist, separate from the "edit chat".