The answer comes in a form of a drop off each of the bosses inside the instance: The Essence of Immortals. With this essence, the mages of the Shattered Sun Offensive are able to bring down the magical gates inside the Plateau, three in all, that block access to Kil'jaedan himself.
When you first arrive at the flight master in Quel'danas, you'll notice a ship to your immediate left. Up on the stern of the vessel, You'll find the Archmage Ne'thul, who will describe to you the problem of the barriers. He asks that you bring him Essences of the Immortals so that he and his mages may break down the barrier. To get an essence, you'll have to get 25 people together, and go take down one of the bosses of the Sunwell. The only three you can access at first are Kalecgos, Brutallus, and Felmyst, but all three should drop it. When you bring it back to the Archmage, he'll give you a Cache of the Shattered Sun, but more importantly, the barrier will be closer to being destroyed.
At last check on the PTR, Ne'thul says that the first barrier, Agamath, is down to 46%. Right now, estimates are that each Essence of the Immortals will bring the barrier down by 1.25%, which means that each server will need 80 total Essences - that is, 80 kills of the first three bosses - to bring down the first barrier if Blizzard doesn't change it between now and when 2.4 goes live. Whether we'll need more or less for the next two gates, Rohendor and Archronisus, remains to be seen.
80 boss kills may seem like a bit of work, but luckily, there's more than just unlocking more bosses as a reward for the turn-in. The cache itself is quite lucrative, as you can see from this screenshot from MMO Champion. A good selection of flasks, epic gems, and some of the new trade skill patterns (see fellow blogger Alex Ziebart's series of reports on those) can be found in it, as well Hearts of Darkness, which are used for various Black Temple crafting recipes, and should be nice for guilds that have left Black Temple behind (or skipped it) and want to focus on Sunwell content, but still want some of the great Black Temple crafted gear.
Between this and the progressive dailies for Quel'danas, it really does seem like Blizzard's outdone themselves for this new world event. The unlocking methods hearken back to the old days of the Ahn'Qiraj opening event, but definitely have their own flavor, and it should be great to see servers coming together to build up the Shattered Sun Offensive and clear the way to Kil'jaedan. It'll be fun to see which servers manage to unlock the whole thing first, especially with word going around on the PTR that some servers have people leveling multiple characters to 70 just to get in more daily quests.