Never had a chance to raid in vanilla and only slighty in BC but the sound of a 40 man raid is awesome. That is assuming its done right.
If one was included in 3.3 (which it wont be so dont flame this is hypothetical):
I reckon they should be set up as "elite progression guild only type raids" meaning they should be "unpuggable". By unpuggable i'm talking significantly higher difficulty then LK Heroic 25 and that to advertise in trade for it would basically mean that the group would be fail.
Such an undertaking would only be considered by guilds with a heroic LK "B team" that could consistently clear.
In terms of loot i dont reckon it needs to be that much better than say LK Heroic maybe a minor stat upgrade but basically the same thing.
I agree that the loot wouldnt fit the difficulty but thats the point your not doing it for loot your doing it so you can have an achievement that says "look at me i'm so uber, i put the time and effort of getting me and 39 friends Heroic LK gear then learnt each others play styles to perfection and spent weeks wiping on this 40 man raid to perfect a strategy and finally killed it".
This way it won't take away from people raiding the heroic 25 for gear but will still appeal to those wanting to class themselves as "elite".
Casuals lose nothing as the gear isnt all that much better and those that qq that LK Heroic 25 is too easy will always have something to strive for.
Any thoughts, comments, reasons why this would not work?