Coming out of last night's alpha build is a new section in the "System" settings. I usually don't go nuts over such minor details in the game, but this one has the potential to be huge.
Come Warlords of Draenor, you'll be able to configure graphic settings just for raid encounters. This is a huge thing to add to the game, given that for most people it's very unwise to raid with the settings turned all the way up. Questing, milling around in town, etc... that's all fine to have settings at ultra. But in raids? With 25 other people doing tons of stuff, it's best to back it off a bit.
I often just leave my settings in-place for raids and don't change them when running around doing other stuff. I like to just set things and forget them. This feature will allow more people to experience Azeroth in its grand detail.
Best feature ever? Yes.