As you might guess from the name Elitist Jerks is not a site promoting casual gameplay. So I was a bit surprised about this article on raid sizes, which is a very reasonable and well thought out analysis of the subject. And I admit that I was one of many who thought that a 25-man raid would be more casual player friendly than a 40-man raid, and got proven wrong by BC.
Good quote on why this could be: Perhaps it is more difficult to design encounters where each player is now twice as important – perhaps further segregating skill and execution level amongst diverse raid guilds. Especially when there are no "easy" raid dungeons around for the other half who isn't as good in "skill and execution".
Also there is a surprisingly honest quote about what this skill and execution really is, and why it is hard: At some point ... skill becomes a non issue – and “knowledge” is all a small gifted guild really needs. If walking in a circle was the only execution based element to a fight, it would be soloed 90% of the time by a single player. But have 40 people doing this in concert, and you have one of the hardest fights in the game (Thaddius – uh oh, time for a graph… 0.9^40!). The concerto of players, and the potential elements of larger numbers must be a factor in this simplicity. 0.9^40 is just over 1%, so any activity that could succeed 90% of the time, if you require 40 people to do them at once lowers your success chance to 1%. With 25 players you have a 7% chance of success, which suggests that a 25-man raid would be easier than a 40-man raid. But in fact in a 40-man raid there were probably only around 25 people or less who actually were required to execute perfectly, the rest was slack. And as the slack could add or take over a function of somebody absolutely required, 40-man raids were easier than 25-man raids now. The "slack" is necessary, because you can't exclude the possibility of somebody having a sudden Real Life ® interruption or losing connection. If one priest in a MC raid got disconnected, the rest could continue. If one priest in a Karazhan raid gets disconnected, the others wipe.
The only way to make 25-man raids more casual player friendly would be to make them easier, so that they *could* be done by 15 perfectly focused players, and the average guild can go there with 15 reasonably focused players and 10 not-totally-necessary players as backup.