This encompasses more than just one bug report of a missing NPC or such, and I'm wondering if I'm just missing something (like maybe this zone wasn't developed at this point in the Classic lifecycle).
Felfire Hill, Demon Fall Ridge, and the attached Barrow Den are not implemented. There are a smattering of demon NPCs on the hill and a few more on Demon Fall Canyon, but not nearly in the numbers they should be, and the quests (Destroy the Legion, Never Again, Diabolical Plans) cannot be completed because the NPC in Splintertree does not exist to begin the kill quest and the plans do not drop (or if they do, they cannot be turned in anyway, I only killed a few enemies but the missing Splintertree NPC is still necessary). The barrow den is entirely empty, I believe.