7.0 will bring, among other things, demonic invasions to Azeroth. These are limited to only six zones, but the events are pretty fun and reward really nice gear. You're guaranteed to get two pieces of item level 700 armor if you stick it through a whole invasion, and occasionally you'll get a weapon that starts at 700 but can be upgraded with "Crystalized Fel" five times to eventually get to item level 725. (If you don't get a weapon, don't sweat it too much, as the first thing you're going to do in Legion is get your artifact weapon.)
The invasions have four stages, at the end of which you'll get 10 Nethershards, or 20 after the final one. After stages two and four, you'll get chests that contain armor and a chance at weapons or crystalized fel. There are Illidari camps in each of the zones where invasions are going on as well as Stormwind and Orgrimmar, and you'll be able to purchase a number of things with Nethershards, including Crystalized Fel, a Felbat pet, full Appearance sets for each armor type (I believe this is the same look as the stuff that drops from the chests, but you'll be guaranteed to get every piece,) and a few other things including level 700 necklaces, cloaks, rings, and trinkets.
Invasions occur in Westfall, Dun Morogh, Hillsbrad Foothills, Azshara, Northern Barrens, and Tanaris. Each invasion will start with a localized attack on the zone's central town (Dun Morogh uses Kharanos while Azshara actually just uses the area in front of Orgrimmar's northern gate.)
Phase one involves waves of demons attacking these small areas, with very few elites mixed in.
Phase two has two or three demonic lieutenants show up, followed by a demonic commander. These guys are a bit tougher, but should go down pretty quickly. Killing the commander will yield a Small Legion Chest, which has 10 Nethershards and a piece of armor inside.
Phase three opens things up significantly, and lasts the longest. Demons will appear throughout the zone, along with lots of demonic buildings. These buildings can be destroyed by smashing the fel crystals supporting them. The smaller forts simply have an unprotected crystal inside. Fel gates have two crystals that are being protected by Eredar demons who channel an immunity spell on them. The larger fel fortresses have three crystals that protect the front door, and once these are gone, you can go in and kill the fort's commander, which will destroy the fortress.
Throughout the zone during phase three, there will be many boss-level demons that can be killed for a chunk of progress on the bar, but these guys will hit quite hard, requiring something akin to a full dungeon group to take on. You'll also be able to rescue NPCs, some of whom will join you in combat for a few minutes. Additionally, you should be able to find various siege vehicles around the zone, though in my experience their damage has been low enough that they're really more useful for keeping you alive and giving you long range attacks.
Phase four will see the initial invasion site attacked by a large demonic general. These enemies are more or less full-on raid bosses, and even if you have a tank, you will need to keep them healed constantly and also respond to extremely unforgiving mechanics. Expect many, many corpse runs when dealing with these guys.
When the general is slain, you'll receive a Large Legion Chest with a piece of armor, 20 Nethershards, and potentially a weapon appropriate to your spec or piece of crystalized fel.
On the PTR, these invasions are happening pretty much constantly in all six zones (though not Westfall recently,) but I suspect that on live they might instead happen one at a time. These are definitely designed to have a lot of people participating in them.
Also, they're good for both factions. While the invasion is going on, local NPCs of the opposite faction will act friendly toward you. So if you're Horde and find yourself in Kharanos or you're Alliance and find yourself at the Crossroads, don't worry about Muradin or Saurfang, respectively, chopping you to bits. They're just happy to have any help they can get. (Each zone has a main NPC who is leading the defense. Azshara has Gallywix, Tanaris has Noggenfogger, Westfall has Gryan Stoutmantle, and Hillsbrad has Helcular.)
Not only are these invasions a lot of fun and reward really great gear to get you into the new expansion, they are also a great place to test out the changes to your classes and specs. Just remember that you've got to be out of combat and in a rested area like an inn if you want to change talents.
I'm hoping/assuming that these invasions will continue after Demon Hunters have been unlocked, and so I bet this would be a really great way to get a handle on the new class.