It's what Blues mentioned that they'd like some kind of conditional progression.
Onyxia requires you to go on a killing spree all over the world.
MC requires you to go back to BRD and plow your way through basically two thirds of that instance and fondle a rock.
BWL required you to go back to BRS and plow your way through all of the upper part of that instance and fondle an orb.
Not only that, but there's a more subtle prereq issue going on that's not even that apparant at first glance but really required to actually finish an instance.
MC requires you to grind faction with the Thorium Brotherhood to get FR gear (Flamepacks, Ragnaros).
BWL requires you to grind faction with the Thorium Brotherhood to get FR gear (Vael).
BWL requires you to kill Onyxia repeatedly for Onyxia Scale Cloaks (the three Drakes and Nefarian).
Now, it would only seem logical that there's some kind of prereq for the Ahn'Qiraj Temple (40man raid)...
...that forces you to kill a truckload.
...that forces you to grind rep.
...that forces you to have more or less completed an "earlier" instance.
Considered to be only a "AQ Door Opening Quest" at the moment that's there for the uber guilds, there's actually a brilliant comment in that thottbot link mentioned here.
Blizzard designed what seems to be a Pyramid Scheme for reputation grinding - read the comments behind the link.
You have one person gearing up everyone else as a "Deputy" in order to collect more crap in order to "Deputize" more folks.
It could be... the attunement quest to get into the Temple of Ahn'Qiraj?
