Every so often someone looks back to 40-man raiding: this time it's Zarhym on the official forums, answering a question about raiding's future and their possible return. I have to say that every time someone decides that the 40-man raid game should come back, I cringe a little bit. So Zarhym's answer works for me.
I agree that there were some very epic moments in the old raid game: last night for instance I was looking at old screenshots of myself from those days and gabbing with the people who work here about our respective accomplishments and experiences in original WoW raiding. The great C'thun death beam chain ride, where we all mounted our bugs and rode in at once to see how high we could get it to go (I believe I took 6 million damage, with another 6 million resisted), the night I got my Sulfuras, the Onyxia attunement chain and what an incredible pain it was to wander around Desolace looking for Rexxar.Right now our plan is to look back on 40-player raids fondly as a format by which we were able to evolve and customize what we feel to be the best raid formats for World of Warcraft. I don't believe our developers have ever stated that they'll absolutely never do a 40-player raid again, but we're really happy with the raid variety we have now with 10- and 25-player raids featuring normal and Heroic difficulties.
I'm a veteran of the 40-man raid game from both the Horde and Alliance side, and I have to say, I really only miss the experiences through the patina of time buffing up all the glory moments like killing the bosses for the first time and letting you forget the pure hassle of getting 40 people together. The griping over having to break up into 15-man groups to go run UBRS and MC the mobs there for a fire resistance buff for Vaelastrasz is long since forgotten. I think in the end 40-man raiding was cool and fun and an artifact of the games that preceded WoW, and it belongs in its place of honor but definitely in the past.