While some players are keen to enter the era of 15-man raiding, others loath the roster moves and guild shakeups this would cause. For those that were not around during the 40-man to 25-man transition in The Burning Crusade, many many guilds fell apart over having to make drastic cuts from their rosters. In a series of tweets, Ghostcrawler notes that the transition to 15-man raiding would cause guilds to prune 10 people from a 25-man roster, however I'd contend it'd be much more than that (around 15 people would need to be cut), due to the fact most 25-man guilds carry several more folks than necessary, which wouldn't be needed in a leaner and meaner raiding crew.
However, it appears that a transition to 15-man raiding is essentially off the table. Ghostcrawler tweets:
@_jkbates We did and it sucked. Many players never forgave us. And the raiding population is much larger now. It's a last resort option.- Greg Street (@Ghostcrawler) January 25, 2013
Have players forgiven Blizzard for the social shakeup that came with WoW's first expansion? I'm not sure to be honest. Obviously many people agree that 10-man and 25-man raids are logistically much easier to get running, but the initial social cost of that jump was pretty high. Personally, I'm over the transition -- it happened long ago. But, it's clear that Blizzard learned their lesson about changing the social structure of the game so drastically; they don't want to do it again.
The 15-man raid being an option of last resort also puts a nail in the coffin of those that thought Ghostcrawler's "unannounced feature" was a conversion away from 10-man and 25-man raiding.