One size fits all 20 men mythic raids. Back then I was apprehensive, it did kill my guild at the time when we failed to go from 10 men to 20, but in hindsight I can now see how much better it is to have a unified finish line for cutting edge raiding. I still think it's not being used to it's maximum potential, haven't seen enough mechanics evolving around cool class abilities (the priest mind-control was a common example when they implemented this size) but I do prefer it over what we had before.