From what I have read it will be a pick and choose type thing where people will go after the easier (entry) bosses til they get some gear, then go for the intermediate bosses in whatever order they personally find easiest, then attack the final bosses who will all roughly be of similar difficulty.
Once people start to clear whole raids they will simply go round them in the most efficient order for each group.
Long story short, they will all be roughly the same difficulty in terms of gear required, thus each group can pick and choose whatever order they want.
Their all roughly the same difficulty and you would likely be attempting all three each week, though the hardest boss, the heroic-only lady Sinestra, will be at the end of heroic bastion of twilight.
Yeah, I reckon Bastion of Twilight is where the real "race" wil take place. I mean, sure Al'Akir, Neferian, Cho'Gall will all three carry some prestige (for lack of a better word) with them.
But I think the order of coolest bosses would be Sinestra>Nefarian>Cho'gall> Al'Akir.
In general Isashi is right though. Prett much like it was in T6-> You had all the 4, 5or 6/9 BC, 4/5 Mount Hiyal guilds. Then people would go back for Archimonde. On the other hand, my guild found Archimonde a joke, and had trouble with anything past Akama at first.
My understanding is that Throne of the Four Winds is the harder of the dungeons (this being intended is unknown) and Bastion of Twilight is probably the easiest.
Its actually different on hard mode, also Al'akir and Cho'gall are by far harder than every other boss.
So Id say the progression will be very weird. First, guilds might go for the first boss of each instance. This is then followed by 3/4 of Twilight Bastion, then 6/6 on blackwing descent. Then Cho'gall and Al'akir last.
As for HEROIC:
3/4 h twilight bastion 2/6 blackwing descent 1/2 Throne of four Winds bosses are decently hard
Most Blackwing descent bosses seem like they are going to be fairly challenging.
Hardest heroic bosses seem to be: Atramedes, Cho'gall, and Al'akir.
From what I gather it's going to be a cluster.
To compare Cataclysm to the likes of Burning Crusade.
All of the Cataclysm raids will be similiar to SSC/The Eye.
They will all be done at the same time.