Plaguewood is a large area directly south of Stratholme and stretching all the way to Terrordale within the Eastern Plaguelands. It is heavily occupied by the Scourge and contains large amounts of infrastructure, including ziggurats and slaughterhouses.
Before the coming of the Plague, the region looked very similar to Elwynn Forest. However the area has been drastically altered by the Plague of Undeath. The area is presumably named Plaguewood because of the drastic effect the Blight has had on the landscape. The soil itself resembles a sponge in appearance, and the ecology of the area has been dramatically altered since the days of the plague. Fungus grows to amazing proportions, predominantly in the form of large mushrooms or toadstools, dwarfing nearby trees. It often belches out dark brown gas, looking similar in appearance to the gas that spews from Plague cauldrons; whether or not these fungi are a tool used by the Scourge to spread clouds of noxious plague gas is not clear.
Plaguewood was the home to an imposing structure that opened a portal to Naxxramas, the enormous necropolis of Kel'Thuzad that once floated above the forest. Access to this portal was controlled by the Argent Dawn. Only those who were exalted with the Argent Dawn could enter free of cost, while others had to pay a hefty fee. (The quest in question, remained in game until Cataclysm and patch 4.0.3a.) In recent times, Naxxramas has left the Plaguelands after being summoned to the Dragonblight in Northrend. The teleport device to Naxxramas has since been replaced by a different structure.
Despite (or perhaps because of) the prevalence of the Scourge, representatives of the Scarlet Crusade routinely patrol the road leading to and from Stratholme. They attack any undead that they encounter — as well as any other travelers that these fanatics chance upon. All those journeying along this road should be on the lookout for them, as they present as much of a danger as the ravening Scourge.
Its original name might have been Greenwood, as the Thalassian Pass that unites the Ghostlands and the Eastern Plaguelands (that is in close proximity to the forest) could have been the Greenwood Pass seen in Warcraft III.
Map of the original Plaguewood in the Culling of Stratholme instance.