I don't know what it is either...
While the LA2 concert I went to was a good one, I listened to it later and was stunned by how quiet the audience seemed.
My section (which was lower level, right across from the stage) was rockin' (everyone, including myself, was singing along and dancing the entire time), but I remember points in the concert when Bono had to sort of keep the audience singing... (during Pride and/or SBS, I believe). I remember it because it bothered me - I don't understand how you wouldn't want to sing along...
Now, I know the recording does cut out some of the audience participation (I was there, afterall, and people were singing), but if you listen carefully, it's obvious that the audience gets louder on the choruses of their better known songs. When 'the Ocean' came on, it got eerily quiet, which tells me that a lot of those there perhaps only knew the "bigger" songs... Yeah, it's not exactly a crowd-rousing song, but still... You could tell people didn't know the words to 40, even.
Regarding 40, specifically. I think it is a bit odd with everyone walking off stage... I don't know if people get what's going on... Larry pauses before beginning his solo and I think that throws people off -- singing '40' while he's jammin'? While no one is on stage?
It was a bit awkward... When he walked off, some were still singing, but people were busy cheering and applauding...
The house lights coming on quickly didn't help matters, either...
I'm not saying it's an excuse, but it might be part of it...
Of course, I don't have anything to compare it to (it was my first show), but, I think the recording doesn't exactly do the concert justice...
Plus, while the audience might not have been singing (for whatever reason), everyone whipped out there cell phones when Bono asked, laughed when he talked about the Pope, stood up, and danced, so I don't think it was audience apathy or anything...
I just don't know why they wouldn't sing more enthusiastically....
Ok, I don't think that made anything any clearer... :/