Tsieng Liu from the LotRO Dungeons Team describes Dol Guldur as "the fortress where Sauron dwelt when he was known as "The Necromancer." It's an old fortress that is now full of the foul servants of the Enemy on the edge of Mirkwood forest. It is in this region that the elves of Lothlórien are mounting an offensive in order to keep the Enemy from penetrating their forests."
Dol Guldur will contain three 3-person dungeons and a 6-person dungeon, leading up to a 12-person raid in which you storm the tower of Dol Guldur and take on the "leadership behind its forces." This area will also feature some fun new mechanics and challenges that should turn your tried-and-true group methods of playing neatly on their ear.
Dungeon overviews, as well as a look at the new surprises Dol Guldur has in store, can all be found in the diary entry. It's well worth the read.