We are going to be unlocking the first wing of Siege of Orgrimmar, Vale of Eternal Sorrows, in Flexible mode over the weekend. (source)
Players with a premade group of between 10 and 25 players can queue using the same interface that you'd use for Dungeons, Scenarios, or LFR. Flexible mode bosses have all the same mechanics as the Normal mode versions, but are otherwise tuned to be more forgiving, and will scale in health and damage output with the number of players in the raid. As you go from a 12-player raid to an 18-player raid, for example, some abilities may affect more targets or hit slightly harder, creatures will have more health, and so forth. Difficulty may currently be slightly higher than our final targets, but we're still working through the details of the group-size scaling.
Please provide specific comments and feedback in the dedicated feedback thread.