I say U2 should stop playing "40."
On bootlegs I have from 80's shows, it was great...On the Vertigo tour, it just didn't sound too inspiring. Were talking a more obscure U2 song from what, 24 years ago now? The crowd simply just does not get into the song they way they did in the 80's because a huge chunk of the audience I would bet has never heard the song!
I saw U2 twice in Chicago--On May 9, they ended the show with "40," which seemed like air was slowly being let out of the balloon. Sort of boring, sort of a "meh," from the audience. (Maybe 1/4 of the crowd continued to sing the "How long..." part after the band left.
On May 10, they ended the show with Vertigo--all I can say is wow--It was fantastic.
I personally like when the closing song is more of a hit that everyone knows. For example--When I saw Pearl Jam a couple years ago, they ended the show with "Alive." The crowd was insane---talk about ending on a high note.
Great closers in this same vein for U2 would be "Bad," "All I Want Is You," "With or Without You," or hell, I'd even like to see them go out with "I Will Follow." The only exception to this rule is "Love is Blindness." Probably the best closer that U2 has had.