One Tree Still said:
Good call Typhoon. U2 is at their best when they don't try to appease the mainstream.
In a couple cases, they ended up defining the mainstream because they were so good (Johsua Tree, AB to a lesser extent).
I would not call JT, AB or ATYCLB "mainstream".
JT sounded like nothing else in 1987. That was the year when Bon Jovi and Michael Jackson ruled (along with U2) as well as big hair bands and various pop artists.
AB was vastly different from the grunge sound of Nirvana and Pearl Jam's albums of late '91/early '92. The look, sound and even "feel" of AB was hardly mainstream compared to the rest of the music scene.
With 2000's ATYCLB, rock seemed to be taking a major back-seat to pop, country and rap music. U2 were able to successfully break across the "rock indifference" of recent years and not only create a critically and commercially brilliant album, but a wonderfully-received tour as well (while those pop artists, who dominated the charts, were having difficulty selling out shows).
Many people praise "Million Dollar Hotel". I enjoy some songs, but to be candid, I find it very similar to The Passengers work - some moments of brilliance and a lot of self-indulgence with music far too estoeric for me to personally appreciate.
If JT, AB and ATYCLB are U2's attempts at being "mainstream" then I say more power to them. Because whenever they make these attempts, I find oustanding work that will have an eduring appeal. In contrast, when U2 tends to be too "arty" for the sake of just being different, their work at best is refreshing (from the perspective of seeing a big band like U2 produce these more "experimental" tracks) and at worst is quickly forgotten.
Unlike many fans on this forum who suddenly have this strong, inexplicable dislike for ATYCLB, I found it a brilliant album, well-deserving of all the accolades it has received. I can only hope that U2 continue to challenge themselves - to produce work that is accessible to the mainstream public, but is still clearly something that only U2 could produce.