Vector's unique "Active Camoflauge" ability that renders him nearly invisible. Something about not seeing a guy ready to plunge a knife into your throat before death is kinda unnerving.
USS Command. Unforgiving, cruel, quick to lose his temper, extremely psychotic, and will leave you for dead in a zombie infested city to fend for yourself like a common citizen for being unsuccessful in your mission even if it's not in the slightest your fault.
The new NE-Beta parasite B.O.Ws and Parasite Zombies exclusive to this game. These bugs are essentially the adult form of the same parasite put into the Nemesis to make it under Umbrella's control and follow simple orders. They're pretty gross and creppy by themselves, but then one crawls on the nearest zombie's back, bites off its head, and then commandeers its body like a robotic suit.
One of them pulls that on a TYRANT in a late Spec Ops mission.