I wouldn't say Pop was U2's last rock album, necessarily. HTDAAB is pretty rockin', imo - Vertigo, LAPOE, and ABOY are straight up rock. Vertigo, especially, is one of the most simply rock songs U2 has ever done. Miracle Drug, City, and Crumbs are somewhat rocking as well. However, I do think that Pop is possibly U2's most heavy rock album (well, perhaps second to Achtung) - Discotheque, Do You Feel Loved, Mofo, Last Night on Earth, Gone, Miami... I think maybe the difference between HTDAAB and Pop is production. I know everyone says it, but HTDAAB just sounds so polished and squeaky clean. It's too clean. Too bright. Pop has a ton of production as well, but it doesn't live up to its name. It's not very poppy at all. The album is very layered and has so many sounds going through it, yet somehow it still comes across to me as raw in a way. I can't quite explain it. It's just a feeling. It doesn't really come across as unfinished to me, though. I imagine a more finished Pop would sound like the new versions on the Best Of, which were basically cleaner versions of the songs. I enjoyed the new mixes, but I think they took away a certain atmosphere that hangs over Pop.