Blizzard is once again tweaking the way raid difficulties work, first trying through achievement mechanics, then through entirely separate zones, and now a third experiment with Icecrown Citadel: a difficulty setting you can change on the fly through the user interface. Check out the details below, provided by Bornakk!

Quote from: Bornakk (Source)

In the upcoming patch we are adding a new feature to the Icecrown raid instance that allows the raid leader to change the instance's difficulty setting on a boss per boss basis. The way the raid leader chooses to switch is the same as now, by right-clicking on the character portrait. The difficulty settings can be changed from inside Icecrown Citadel, with some restrictions. For example, you cannot change the difficulty in combat or during certain events specified by our design team. To enable the ability to change the raid difficulty to heroic, you must first defeat the Lich King on normal mode, as doing so unlocks the heroic version of this raid instance. There is no heroic version of trash; meaning changing the difficulty will only affect boss difficulty.

We chose this particular functionality because we didn't want to use the Trial of the Crusader method, and have four versions on a raid of this size. We felt the Ulduar method of having to know a certain trick to do on the boss was difficult to communicate and tied too heavily to achievements. We might eventually convert Trial of the Crusader over to this new system but Ulduar and Naxxramas will likely never change.