Halls of Stone surprises me somewhat, as I really didn't think it was a difficult heroic (outside of running a Druid tank on the Brann Bronzebeard event), but Ahn'kahet is universally cited amongst tanks and healers for LOS issues and nasty damage. And Oculus...well, we've talked about Oculus here before. I still think the concept of the dungeon itself is fantastic, but the moment that one or two embarrassed people in your group admit that it's their first time on the drakes, you know you're in for a long and ugly run.
I hope Flyv revisits this data at some point, because I'd love to see how the numbers change over the course of Wrath. With achievements introduced only as BC was ending, we don't really have a good look at how dungeon popularity and completion rates evolved there, and I think the numbers say a lot about the content players prioritize when they get a toon (or multiple toons) to the level cap.