Hey, you should get Kurt on the smilie List
I'm taking voice lessons (not that this makes me an expert, but just to give you an idea) and I've tried choosing a couple of U2 songs (including Beautiful Day) to sing for performances, and wound up giving them up because of their difficulty. It's not just the notes, timing, sequence, etc. that make up a song?s difficulty, but its range. Bono has an incredible vocal range, believe it or not, and he uses the entire range within the song, which is what makes things tricky. In singing, many people have a head and a chest voice, for high and low ranges respectively. This alone isn't hard, but where people encounter trouble is in the mid ranges, because you either have to sing really low with your head voice (super hard) or really high with your chest voice (also hard). Even more difficult is transitioning between head and chest voice, which most people just plain can't do. In fact, what makes singers like Ella Fitzgerald so amazing is that they can bridge this "mid range" flawlessly. Thus, most songs are written for either head or chest voice.
Many of U2's songs, though, (including Beautiful Day) include high and low ranges, and so for Kurt to sing the entire song, without you hearing him switch from head voice to chest voice or vice versa, was really very accomplished and challenging.
Furthermore, one might consider that part of being a good performer is making what you do look easy. So in that respect, Kurt is just doing his job.