The good thing about it is that it doesn't rely on monsters hiding in cupboards, or dark crawlspaces. Quake's horror is in its completely 'other-world' feel. The fact that there are no kitchens, bathrooms or fire exits suggest that the inhabitants have no purpose other than to guard these vast grandiose chambers that in turn seemingly have no other purpose than to be guarded. The death knight standing in wait behind the next door may have been stood in that spot for hundreds of years without moving an inch. Dust has probably collected in thick reams on his armour. Then one day the door opens and without a second of hesitation he lunges at you. The enemies are completely single minded in their desire to kill you, and you don't even know why they want to do that in the first place!

It's scary because it's inexplicable.