One thing that's really different now than in the past is the internet. People can check out the setlist for that night's show and critique it within minutes or hours after the show (sometimes even while the show is in progress!), thus everyone has an idea what the setlist will be by the time the tour rolls around to their area. I think U2's relatively static setlists probably used to be justified by saying they'd rather focus on getting a relatively small number of songs really tight in performance, rather than shifitng it around a lot and being unprepared - kind of like a fixed travelling show who's quality and reliability would be great. After all, if you saw a travelling production of say, Hamlet, you wouldn't expect the actor to ad-lib some new lines each night just because you've heard the same ones before! In the pre-internet days they could do this and most people wouldn't have known what last night's setlist was. It was always fresh for the new audience. Now, with setlists (and even recordings of shows) easily available online, it comes to look predictable by the time the tour gets to you.
I remember ZooTV - way back in those pre-internet dark ages of 1992!. I didn't see it until September, the third leg of the tour that started in February in the US and then went to Europe for the Summer, before coming back to the US. Despite "my" show coming so late in the tour, I hadn't heard a single recording of any ZooTV show or song live before that, nor - aside from reading a few reviews that gave most of the setlist and thus having some idea what songs they might play - did I really know what the setlist would be. Though obviously I knew new Achtung Baby songs would predominate, I had no idea what order or what they would sound like live. It was all a marvelous surprise.
Fast forward to 2000: completely different. Not only had I been able to download a couple of recent shows before I saw them live, I had seen all the setlists online, knew almost exactly what songs they would play and in which order (only one song was different than I expected), and at a bar before the show I was actually telling a few people the name of the closing song in the setlist. Though they had actually seen the previous night's show, they didn't know what the song was called!
Quite a change actually in a few short years, and I'm not sure the band themselves (and other bands and artists too) really even realize yet how the quick availability of setlists and bootleg recordings online affects fans' perceptions of their recent performances and setlists.