Rohan over at Blessing of kings posted recently about the slow death of 25-man raiding Guilds in World of Warcraft. He points out, that most guilds would not bother to raid 25-mans if they are able to establish successful 10 mans. I think the difficulty in locating enough people to do 25-man raids as opposed to 10-mans is the main culprit for this decline. Pre-Cata there was an abundance of people willing to raid and nothing to stop them from joining a new raid guild. Today however there are less and less players to choose from and few of them willing to jump ship.
In a previous post, I wrote how I felt this is a direct cause of the new Guild Advancement system. Imagine for a moment that you are forming a new guild, raiding, casual or whatever. Now your options available to you would be to either collect a new group of players just starting out (which I question just how many actual NEW and not alts there are out there) and leveling together or form a new guild from existing level capped players. Most players today are reluctant to leave their present guild because they don’t want to lose the perks and guild rep they have established. This alone will drastically cut the amount of applicants you can talk to as compared to WoW before Cataclysm.
So with this information in tow, imagine now forming that raiding guild and trying to attract the necessary people to do it. Will you even bother to or even be able to attract enough members to form a 25-man team? I believe that not only will we see the demise of 25-man raiding but large guilds for that matter. As the old guard guilds begin to die out a few will reform into new ones, but they will most likely be centered on much smaller groups.