One more thing that comes to my mind when looking back at the progression in TBC versus vanilla WoW and one specific problem and different in TBC(not sure if it was mentioned already in the thread, but haven't read a post about it):
Due to the current attunement chain that Blizzard has in place raiding guilds are now split into 2 big camps:
T5 raiding guilds who are still working on Lady or Kael.
T6 raiding guilds who either have or probably soon will have MH and BT on farm.
For simplicity let's assume that the T5 raiding guilds will keep banging their heads on Lady and/or Kael for the time being until Sunwell comes out (some guilds might kill them, but others will "take their place" so to speak).
The end effect of that will be that the average item level a T5 raider is wearing will hover somehwere around 133 (ilevel of T5) whereas the T6 raiders will at least be at 141 if not higher (exalted rings/trinkets as well as Illidan/Archimonde loot).
Thats a gear difference from an item level point of 8 level already. Add to that that T5 raiders probably won't have access to MH/BT gems and won't have either chest or head pieces this will overall probably add up to 9 or even 10 item levels.
Now Blizzard has a problem with the tuning Sunwell: How hard do they make it? If the attument chain isn't linked to MH or BT attunement they will need to tune Sunwell to roughly itemlevel 133. This will then though mean that all T6 farming guilds will just blow through that instance in a matter of weeks as their gear already is superior to the tuned difficulty of the new instance.
If Blizzard on the other hand choses to link Sunwell attunment to MH or BT attument and tune it for item level 141 they will effecively lock out all T5 or lower raiders. Even if they don't link the attunement to MH or BT, if the encounters are then tuned for item level 141 those T5 raiders will simply bang their heads against nearly impossible encounters for them. And due to the consumables fix that Blizzard did (and thank god they did!) it's not possible anymore to just beat an encounter by overuse of consumables.
So Blizzard effectively steared themselves into a dead end with the current attunement chain from T5 to T6 content and will either make the game boring for T6 raiders or impossible to reach or beat for T5 raiders, thereby locking out a large part of the raiding community. This hasn't happened in vanilla WoW where virtually every raiding guild could just go to any of the 40 man instances and find their level of difficulty.
Maybe, just maybe a solution would be to open up MH and BT 1-2 months before Sunwell comes out so that those T5 raiders could at least begin to gear up a little bit despite not having finished T5 content could then prepare themselves a bit for Sunwell and have a slight chance.
And possibly make the difficulty progression in Sunwell a bit steeper than it was in T5 and T6 content, so that the first 1-3 bosses would be tuned more around item level 133 whereas the later ones would then require 140 or more.
I'm really curious how Blizzard will handle that issue.
This isn't an issue, this is progression. If you can't handle fights like Vashj and Kael how do you expect to be able to handle a zone that's supposedly harder than BT/Hyjal.