I don't think this even needs to be said, but huge huge spoilers ahoy.
With patch 5.4 going on the PTR with the quickness, we're starting to see just who, exactly, we're going to be fighting in the Siege of Orgrimmar raid, and it's definitely not as straightforward as we might have thought. With the folks at Wowhead having the zone page and Dungeon Journal information up, we can take a look at what's to come. Be warned, there are spoilers for the upcoming raid in plenty past this link. If you don't want to know what the bosses will be in this raid, then do not read past this point.
Okay, now that they're gone, let's gorge on what we can discover.