Pathnoding
Proper lighting
Geometry errors (invisible holes in walls)
Zoning
BSD cuts
Teleports to next levels

House:
New lampposts
Toilet
Sink/mirror
Bathtub/shower
Car(s?) outside
Stereo (or just a clone of our boombox/CD holder)
Laptop on living room floor?
Automag/ammo?
Translator
Static noise from TV
Ceiling fan hum
Tracker music