There are about three hundred million points, strictly musically speaking, to be made about U2. One of the main ones of interest to me is that it seems, on the surface, like they pretty much crap hit songs. I've seen (and heard) direct and anecdotal evidence, however, that they actually work very hard to make it sound like that way--recording full demo versions of songs they're working on, listening to them, taking them apart, figuring out what's working and not working, and putting together a new demo. Rinse, lather, repeat. Lesson 1: there can be an enormous amount of effort put into something that sounds effortless. Sub-lesson: making something sound both effortless and, you know, good is almost always hard.
There are, of course, really two U2's. There's the studio version and the live version. As discussed long ago in this very forum, the latter is literally a communal religious experience, and while I suppose there are ways listening to an album can be transcendent in its own way...well, anyway, that's outside the scope of this discussion. The point being that there are songs that are great recordings and don't translate as well live ("Beautiful Day" being an example, so I'm told) and vice versa (most of the rest of their canon I gather, Bono being Rock Jesus and all).
Here's what U2 is all about, in my opinion: space. Compared to almost all other popular music, there's comparatively few parts making up the song--bass, drums, lead guitar, vocal, maybe one harmony. Very few overdubs (our current example has only a little bit of low, rumbling synth in the middle/end, which I defy you to claim that you would have noticed if I hadn't pointed it out). So, on to the construction of "Exit": First verse: the bass and Bono singing breathy, right up into the mic. Second verse: the same thing, with Bono opening up and singing louder. Some drum hits and guitar filigrees (The Edge is all about the filigree, don't you think?). This is a four minute song (I'm talking about the album version now). This band has three instruments total. For the first minute and a half, only one of them is continuously playing. Then they song kicks in for 45 seconds, and then everything but the bass is gone for another 45 seconds, and the rest of the song repeats that basic pattern again. To make a hack-jazz comparison, it's all about the notes that aren't played.
I think space in music is both literal and figurative. The literal is constructed (at least) of both gaps in the sonic spectrum and with reverb (I read in an article about mixing that you can think of sound two-dimensionally with the stereo pan moving a sound left-right and a reverb pan moving it forward-backward. Things with lots of reverb sound farther away). The figurative is whatever it is that lets the listener into the song--metaphorically speaking there's room for him or her to get into the song. That works strongly here--when the song really starts to rock, you've already been sucked in to the atmosphere and it's that much more potent.
I have a theory that the transcendent listening experience happens because of space, for a definition of "space" that I haven't quite filled out. I actually think there's no real meaning conveyed from the song writer and/or performer to the listener, that instead what's happening is that the song creates a space for the listener to insert his or her own life/experiences/humanity/whatever. That is, the listener is doing all the work, he or she just doesn't know it. You like a song before you know what it's about, in fact you've probably constructed your own notion of what it's about, whether you realize it or not. I always feel kind of sad when I find out what songs are actually about--it's like going from the book version of something to seeing someone else's version of it rendered on film. The picture painted by the song writer is always a little smaller than the one in my head.