At the end of Vanilla, all raiding was done in either a 40 man format (MC, BWL, AQ40, Naxx40) or a 20 man format (ZG and AQ20). Enter BC raiding. Those guilds then had to break up their raiding teams into 10 man groups to run Kara, which was mandatory for access to later content, then they had to rebuild to a 25 man raiding group to tackle any other raids.

Players who wanted to blitz through Kara were upset if their guild built even raiding teams and slowed 10 man progression in favor of having a viable raiding team.

Guilds that did make a ten man all star team and bulldozed Kara hit a wall when they got to Gruul's or Mag, because while those ten were ready, the other fifteen were even worse off than the guys in the other guild.

Add into this the rediculously overtuned content spurred on by blind buffs to all raid content after Death and Taxes rather disdainfully kicked over Gruul on the Beta ptr, and you have a recipe that killed off probably around 70% of the guilds on my server, which was a well progress, high population server.