We all know, or should know, the story of the Draenei by now. It's been beaten into our heads in many different ways in the World of Warcraft. If you don't know the story, let me sum it up for you: About 25,000 years ago, Velen, Archimonde, and Kil'jaeden were the top cats of a race called the Eredar. Sargeras approached them with promises of power and whatever else, with a hidden malicious intent. Archimonde and Kil'jaeden jumped on it, but Velen, being a prophet, had bad feelings about the arrangement. He and those loyal to him avoided falling into the grasp of the Destroyer of Worlds until a savior(...sort of) in the form of K'ure came along. K'ure explained the nature of the Naaru to the Prophet, and Velen gathered the Eredar loyal to him. They loaded up into a ship(later named Oshu'gun by the orcs) and escaped Argus, taking the name "draenei" meaning "exiled ones" in the language of the Eredar.
They rocketed around space for awhile, all was good. Then they crashed on some random planet. That's okay though, it was a pretty cool planet so they decided to name it after themselves. They named it 'Draenor' which translates to "Exile's Refuge." It beats me how 'or' means 'refuge' but who am I to question galactic fantasy languages? When Oshu'gun crashed, the Naaru that dwelled within and powered the vessel were severely damaged or outright killed. K'ure was severely wounded and still dwells within the vessel, sitting on the brink between life and death. D'ore, the other Naaru involved in the crash, died on impact. He was relocated to the area now known as Auchindoun and buried there, the first to be buried in the area later known as the Auchenai Crypts.
If you are unaware, the Naaru life cycle has two stages. Light and Void. The Light Naaru are the ones we know and love, those like A'dal. The 'death' stage of their life cycle is Void, the absence of Light. They essentially become a vacuum, unwillingly absorbing souls and sources of the Light to energize themselves. Players on the Alliance side are never directly informed of this, these quests are almost purely on the Horde faction. Most likely your first exposure to this as an Alliance player will be in Old Hillsbrad. Specifically, Old Southshore. There is a scene inside of the inn where you witness Highlord Mograine purifying a crystal he found in Blackrock Mountain, which will be used in the creation of the Ashbringer. This crystal, which was brought to Azeroth by an Orcish warlock, is not a crystal at all. It is a fragment of a Naaru, possibly even from D'ore himself.
Now that you know the life cycle, it may be a little clearer as to why Oshu'gun became holy ground. K'ure, on the brink between life and death, has not outright fallen into Void but is still unable to control the pull of souls. The spirits of those who have died on Draenor, mostly Orcs of the many tribes at the time, were drawn to Nagrand and the Oshu'gun. The naaru communicated with the Orcish spirits, and manipulated them for their own safety. Through K'ure's guidance, Orcish religious practices were altered in a way that they would care for the naaru and keep him from falling into the void, and destroying the Orcs entirely. Blessed waters and other such things were regularly offered up to the naaru, who was hidden within Oshu'gun, and those offerings were able to sustain him. Orcish festivals such as the Kosh'harg were relocated to the Mountain of Spirits, presumably due to suggestion from K'ure, whether they were aware of it or not.
Years passed, and we've spent the entirety of the Burning Crusade catching up on the history of what happened with the Orcs and the Draenei. The union of the Orcs under the banner of the Horde, the slaughter of the Draenei, the rise of Gul'dan and the sundering of Draenor, all of that fun stuff. Needless to say, Oshu'gun is not exactly the spiritual hot spot it used to be for the Orcish tribes. K'ure drifts ever closer to the Void without the tender touch of the orcs, and the safety of all of Nagrand is at risk.
The Burning Legion, being the lovable bunch they are, recently rediscovered K'ure in southern Nagrand. Agents from the Kil'sorrow Fortress have taken up residence within the mountain and are harnessing the growing void within the naaru to attract, summon, and create void creatures to increase the Legion's numbers. Apparently Orc souls are a reagent in the creation of voidwalkers. Make sure you're taking notes, that may come in handy later. This is good news for the Legion, bad news for... well, pretty much everyone else.
The Horde has a very long and elaborate questline involving Oshu'gun and the Burning Legion's tampering with K'ure, so I strongly recommend going out and doing those. The Alliance, however, has none of these quests, so I strongly recommend you fly to Oshu'gun right now and stare longingly at the mountain, knowing that your faction has been completely excluded from a storyline that is absolutely full of history and lore of the newest members of the Alliance, in addition to massive revelations on the nature of the Ashbringer, the most sought after item in the World of Warcraft. If you don't want to do that, you can hop over to WoWWiki and check out the questline there. It's good stuff.
What else is going down at Oshu'gun? Well, multiple factions of Ethereals want to pluck it out of the ground and sell it. I suppose that's almost the same as holy ground for the Consortium. There's also some sweet markings in the grass around the mountain, but I truly have no idea what they mean. My theory is they exist because they look totally awesome.