I would argue that while ATYCLB was definitely more "safe", it still had some experimental items on it (like "Wild Honey") and even a hint of "Pop" on hit (with "New York"). And it was more "pop" than past efforts (with songs like "Elevation"). That said, songs like "Beautiful Day", "Stuck..." and "Walk On" are more "classic" U2 and the album is a bit of U2 returning to what they knew worked well with not just fans, but the general public.
With HTDAAB, though, I'd argue U2 was already starting to become less "safe". "Vertigo" is a rocker that is a lot different than past songs. "Desire", "Pride" and "Mysterious Ways" are all rockers that would definitely find an audience. "Vertigo" was a bit more out there. The catchiness of the song combined with the ubiquitious iPod commercial helped propel that song, and the album, to great heights.
While HTDAAB did have some safe tracks ("City of Blinding Lights", "Sometimes...", "A Man and a Woman") there was some adventure with tracks like "Love & Peace" and "Fast Cars". So U2 already started pushing the boundaries.
With NLOTH, I'd say U2 are back to "Pop" like territory. No, the songs aren't like "Pop", but they definitely aren't "safe". One might argue that a few tracks, like "Crazy Tonight" are safe - but even "Pop" had some safe songs, with tracks like "If God Will Send His Angels" and even "Please", which fit into more classic U2. NLOTH, like "Pop", saw U2 explore some harder rocking songs (the title track) and different styles ("Moment of Surrender"). NLOTH also had more ambient sounds and music, the type we haven't heard from U2 in a while. Perhaps that is a nod to the past, but it's hardly a safe nod.
So while ATYCLB was U2 reclaiming the title, I'd say since then, they've slowly stopped being so safe and allowed themselves to explore. With the enormous commercial and critical success of ATYCLB and HTDAAB (huge sales, numerous Grammy Awards, etc.), U2 proved they can crank out the big albums. NLOTH was a bit like "Pop" in that it could have been big, but didn't have that one big hit to help propel it. GOYB was similar to "Discotheque" in that it caused a bit of an initial negative reaction to an otherwise good album. That said, if this were 1997, I have a feeling NLOTH would have still sold better than "Pop". Given how little people buy albums now, U2 are now fighting another battle (as is every other musician). Free music is good, but artists need to make some $$ to live.
To relate to my earlier post in this thread, I don't feel U2 are past their peak. But all artists slow down. Unless U2 can come up with a huge smash hit to dominate the download market (like the Black Eyed Peas do), they may have to settle for the more moderate success of NLOTH. Nothing wrong with a Platinum album. It really depends on what is U2's goal - to have that big smash hit or to produce music they love and hope the fans follow. Given the enormous success of the tour and the response to the new songs - even if the album hasn't sold well - clearly fans are following.