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Some thoughts on this thread...
1. U2 is a rock band, plain and simple. Maybe you're looking for U2 to be a "hard (heavy) rock" band which they're not and never will be, and I'll listen to AC/DC when I'm in the mood for "cock" rock. Pink Floyd was not in any way a "hard rock " band, but they were most certainly a rock band. Early U2 was punkish, and punk is a type of rock (though the antithesis of classic rock). I'll take heat for this Coldplay is a form of rock, just a softer (and some might say wimpier though I disagree) form.
2. Why do people have this notion that a lighter song or ballad somehow is not "rock"? . All the rock greats from Hendrix , to the Beatles, to the Stones, to Zep did numerous ballads. ISHFWLF is as much a rock song as Smoke on the Water. Rock encompasses so many sub-genres, including softer ones.
3. Funny how "dinosaur rock" is still front and centre of the rock genre in general; best selling rock artists of late have included AC/DC and Metallica. Kids are still listening to Zep, Queen, GNR etc. Guitar Hero's success was no fluke. No need to retire what seems to work. Whoever said Zep wasn't melodic has never listened to Zep 3. Tangerine was one of their greatest. Hey Hey What can I do which didn't make an album is folksey, melodic Zep at their best.
4. U2 "rocks" in concert, even if that side of them is more understated in studio. Their first five concert songs at one time or another since Pop have been mostly rockers including Beautiful Day, Elevation, Even Better than the Real Thing, Last Night On Earth,Gone, I Will Follow and Until the End of the World. Edge's live solo on Bullet the Blue Sky rivals that of any of the hard rock guitar gods.
Just my take.