Discotheque was TOTALLY different from either Elevation or PopMart (at least how I remember it...).
The encore started with new graphics on the screens, and then there was the PopMusic intro (!). We were going bonkers on the floor. Bono walked around the ellipse to PopMusic, then they started Discotheque.
It sounded a bit like a dance mix (but in a good way), it really gave the show a whole dance club feeling. Until lovey-dovey stuff, it was kind of a dance beat with bass and mimimum guitar (kind of spacey). Then the usual riff, but then for the part where it's usually just the guitars and woohoos, it sort of changed beat, and felt like a slower song.
Hard to explain, but it was AWESOME. Especially given it was the first time they played it. All the visuals were new as well. That explains why it took a while for them to put it in the show, it was a whole new thing, not just an added song in the set-list.