It's not even about that ghost, I'm pretty sure Lisa wouldn't even have made it into the full game. It's about the way it creates amazing tension with incredible audio and subtle elements without resorting to overplayed jumpscares all the time, like when you see Lisa's shadow and hear her breathing behind you, the entire bathroom scene, cries, the radio etc. Everything feels heavy and disturbing. It's a completely different type of fear to the usual "I'm being chased by a psycho/monster and I don't have a weapon" that Amnesia and Outlast relied on and it makes you feel unpleasant.
The best part is even though most of this is scripted, some stuff is not so you don't know exactly when it'll happen (e.g. possessions)
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Also there quite a lot of things that happen in the hallway that go unnoticed.
Outlast was creepy but most of it was jumpscares and I knew what I was dealing with most of the time (that fat
******* or the psychos). This is a lot more psychological and relying on audio, which I love.
Kojima and Del Toro have said they intended to make gamers "
**** their pants" and they might've been onto something, since P.T. was just a teaser and not even representative of the final gameplay style.
Honestly, if someone can get through the full playthrough on headphones or with volume turned up without feeling at least a bit unpleasant/disturbed , I applaud him/her
EDIT: And it's really worth watching through without skipping anything, some interesting stuff happens throughout;p